Defining pornography is culturally and politically sensitive, but it can generally be described as pictures, text or multimedia, of real or imagined events, which are intended to trigger sexual, as well as possibly sensual and romantic, ideas in the viewer. It is a subset of erotica. Some forms of pornography, within specific legal, cultural, and social norms, may also be considered -- or not considered -- obscenity, and thus be illegal.
It is an ancient and multicultural practice. The prefix "porno-" comes from the Greek for prostitute, so pornography can be considered part of the "oldest profession." While some social conservatives appear to insist that it became widespread only with television and the Internet, forms were prevalent in Pompeii,and even more so in Asia. Nevertheless, it is a key issue in current culture wars.
I can't define pornography, but I know it when I see it. — Justice Potter Stewart in Jacobellis v. Ohio
Especially in popular culture, the line between pornography and art is blurred. Madonna's musical performances, for example, always have been "edgy", but her book Sex was legally accepted if controversial. Lady Gaga delivers highly sexualized performances that are also seen as advancing music and visual presentation.
Advertising can be highly sexualized.
Basic Western context
In current Western practice, there tends to be a separation into soft- and hard-core pornography. Very generally, soft-core pornography suggests sexual acts while hard-core depicts them. Legal and business practice further divides hard-core into that which is generally legally available to adults, and into certain subcategories that, such as child pornography or bestiality, which are apt to lead to arrest. The line is not clear, as court decisions have, for example, allowed text-only child pornography as protected speech, but banned photographs since their production would be prima facie evidence of statutory rape or other crime against children.
Another practical Western distinction is that obscenity tends to be illegal, or at least banned from some media. Not all obscenity, however, is pornographic, such as the U.S. example of the Seven Words You Can Never Say on Television, the subject of a Federal Communications Commission action against comedian George Carlin, and a subsequent Supreme Court of the United States ruling, Pacifica Foundation v. Federal Communications Commission Carlin deliberately challenged what appeared to be vague guidance, as television broadcasters, in search of improved ratings, produced increasingly "edgy" content. 
Western countries, however, vary widely about their definition of obscenity. Denmark removed virtually all restrictions in 1969, other than its marketing to children.  Canada had addressed the age-old question of "I know about Sodom, but what is it they did in Gomorrah?" with a definition of a certain class of sexual actions as gomorrahy, prohibiting importation of its depiction until 2003.
Erotic topics are found in some of the earliest graphic representations, as at Pompeii. Gutenberg invented the printing press in 1440, and Pope Paul IV included erotica in the 1563 Index Librorum Prohibitorum, the Catholic list of banned books.
Edo Period Japan
Japan’s Shunga art of the Edo Period (Old Tokyo) is a word for Japanese works of art, specifically Japanese erotic paintings that were made from the 16th to 18th centuries, by reputable painters of the Ukiyo-e (floating world) period. At the time purists of Japanese culture looked down upon Shunga art. While artists thoroughly enjoyed painting erotica, they rarely signed their works, preferring instead to remain anonymous and not harm their careers as more serious, classical artists.
While some Shunga canvases or prints (paintings on wood blocks) depicted sensual scenes of couples in love, others were intensely pornographic, revealing carnal love in all its splendor. Artists exposed the human anatomy in full detail, without any concern for discretion or modesty. However these works were always made with care and in good taste.
Often printed as tiny booklets called "pillow books" or in the West referred to as “sex education”, shunga and related art also served as teaching guides for the sons and daughters of the Japanese bourgeoisie. From the paintings they learned the art of foreplay, sexual positioning, and proper hygiene.
Written in 1749, Fanny Hill is usually considered the first novel-length pornographic work in English.
Erotic photographs have been collected from the 1850s.  The term "pornotopia" was coined by literary scholar and author Steven Marcus to describe the setting in Victorian pornography, specifically commenting on the work Romance of Lust. It is a theoretical fantasy world in which everyone is ready and willing to indulge in all kinds of sexual activity. The term is now in general use and has been used as the title of many pornographic books, articles, websites, comics and videos, as well as being a hedonistic ideal.
Pornography and society
MoralitySocial conservatives often regard pornography as a fundamental threat to society. The Traditional Values Coalition position statement says
The spread of pornography in our culture is a threat to the stability of families and frequently results in family break down, child molestations, and spousal abuse. We oppose this threat because it destroys families and it destroys the person who has become addicted to it. Pornography is a progressive addiction that ruins the conscience of the person. Frequently, this person acts out his sexual fantasies by molesting children, raping girls, and committing other sexual crimes—including murder.
Those of a more liberal or libertarian bent, however, do not accept this threat and consider government restriction of pornography, made by and for adults, to be a restriction of free speech. John Stuart Mill often is quoted: "The only principle for which power can be rightfully exercised over any member of a civilized community, against his will, is to prevent harm to others," with the caveat that only "human beings in the maturity of their faculties" can choose such freedom.  Thus, it is generally accepted, by liberals, libertarians, and conservatives, that "child pornography, which is taken to involve the actual sexual abuse or exploitation of children (with or without their apparent consent), can legitimately be banned in order to protect the interests of children.". 
Feminist authors can be found both attacking and advocating pornography. Andrea Dworkin and Catherine MacKinnon were prominent in categorizing all pornography as an attack on women, while Marjorie Heins and Nadine Strossen variously see it as a censorship issue and an opportunity for positive feminism. Strossen, former president of the American Civil Liberties Union, was a cofounder of Feminists for Free Expression, formed in response to the Pornography Victims Compensation Act proposed by Dworkin and MacKinnon.
Naomi Wolf wrote that Dworkin was right to warn but incorrect about the outcome: "But the effect is not making men into raving beasts. On the contrary: The onslaught of porn is responsible for deadening male libido in relation to real women, and leading men to see fewer and fewer women as “porn-worthy."  A number of sex-positive feminists, however, variously believe that women have a right to pornography supportive of their interests, generally considered to have started with Betty Dodson. Especially supportive are authors that have decoupled gender and sexuality,including those involved in gay and lesbian sex, sadomasochism, transsexualism and dominance and submission, such as Gayle Rubin, Tristan Taormino and Patrick Califia. There are also women who produce pornography, such as Dian Hanson, and those that have been on both sides of the camera, such as Candida Royalle  and Nina Hartley
Impact of technology
The first pornographic movies closely followed the first commercial cinema. Due to the relative inconvenience of projection, the movies tended to be shown in niche theaters, or at clubs. Video tape playback, however, made it easy for individuals to watch them; one cynic said they put pornography into the home, where it belongs. "By 1992, videocassette pornography was a $490 million dollar sales industry." 
DVDs, in turn, displaced video in the pornography market. Prerecorded media, are being displaced by the Internet.
Pornography is a multi-billion dollar industry, and, arguably, is among the most economically viable Internet applications. 
One problem specific to the Internet is the deliberate "mousetrapping" of users, not in search of pornography, to be deliberately exposed to it. A major motivator of such activity is a revenue model in which advertisers pay websites "per click" of a user on their site. 
There have been case reports of compulsive searching for pornography on the Internet.  Other studies found decline in rape statistics, different from any other crime, with the increased availability of Internet pornography. "Using state-level panel data on the rise of the internet, I find that internet access appears to be a substitute for rape. Specifically, the results suggest that a 10 percentage point increase in internet access is associated with a decline in reported rape victimization of around 7.3%." 
Technology further confuses the distinction with "virtual pornography", when it can be demonstrate that no actual child or animal, only computer graphics, were used to create some images. While the U.S. Supreme Court ruled it was permissible in Ashcroft v. Free Speech Foundation (2002), there have been subsequent prosecutions. In Chattanooga, Tennessee, Assistant District Attorney Dave Denny said "when you have the face of a small child affixed to a nude body of a mature woman, it's going to be the state's position that this is for sexual gratification and that this is simulated sexual activity," 
- Todd D. Kendall (September 2006), Pornography, Rape, and the Internet, Clemson University, p. 6
- "Erotic Art of Ancient Pompeii", California Literary Review, 14 February 2008
- Jacobellis v. Ohio 378 US 184 (1964)
- Madonna (performer), Glenn O'Brien (editor), Steven Meisel (Photographer) (November 1992), Sex, Warner Books Inc., ISBN 0446517321
- Federal Communications Commission v. Pacifica Foundation, 438 U.S. 726 (1978)
- Christine A. Corcos (2008), "George Carlin, Constitutional Law Scholar", Stetson Law Review 37: 899-940
- "Denmark: Pornography: What Is Permitted Is Boring", Time, 6 June 1969
- The Unspeakable Sin of Gomorrah: What is Gomorrahy?
- Guy Kennaway (11 May 2008), "Victorian erotica: the original cheeky girls", Telegraph (UK)
- Steven Marcus (1964), The Other Victorians: a Study of Sexuality and Pornography in Mid-Nineteenth-Century England
- Traditional Values Defined, Traditional Values Coalition
- John Stuart Mill, On Liberty, 1859
- Caroline West (5 May 2004), "Pornography and Censorship", Stanford Encyclopedia of Philosophy
- Nadine Strossen, Defending Pornography: Free Speech, Sex, and the Fight for Women's Rights, NYU Press, ISBN 978-0814781494
- Annalee Newitz (8 May 2002), "Obscene feminists: Why women are leading the battle against censorship.", San Francisco Bay Guardian
- Naomi Wolf (2010), "The Porn Myth: In the end, porn doesn't whet men's appetites—it turns them off the real", New York Magazine
- Andrew McBride (2008), Pro-Sex Feminism, The Sex Wars, 1970s to 1980s
- Allison Buchan-Terrell (15 February 2008), "Candid talk with Ms. Royalle", The Gazette, University of Western Ontario
- Nina Hartley (2 February 2005), "Thus I Refute Chyng Sun: Feminists for Porn", Counterpunch
- "Keep Your Eye on the Sexy Adult Industry for the Next Media Convergence Says Adult Video News Publisher", Business Wire, 2 July 2008
- Baron, L., and M.A. Straus (1984) “Sexual Stratification, Pornography, and Rape in the United States”, in N.M. Malamuth and E. Donnerstein (Eds.),Pornography and Sexual Aggression,Academic Press. cited by Kendall, p. 11
- Porn: Business of Pleasure, CNBC, 15 July 2007
- Russell B. Weekes (2003), "Cyber-Zoning a Mature Domain: The Solution to Preventing Inadvertent Access To Sexually Explicit Content on the Internet?", Virginia Journal of Law and Technology, University of Virginia 8
- "Hypersexual Disorder and Preoccupation With Internet Pornography", Am J Psychiatry 158 (10), October 2001
- Kendall, p. 2
- Ashcroft, Attorney General, et al. v. Free Speech Coalition, et al., 535 U.S. 234 (2002) 
- Tennessee man charged in 'virtual pornography' case, CNN, 25 June 2009