Spyware is a program that offers you services like a friendly toolbar for your web browser and then quickly and covertly steals your personal details. It can collect information ranging from your credit card details and the websites you frequently visit, to the ads/links you often click on. Thereafter, it sells this significant information to interested third parties in exchange for a hefty lump sum payment.
Most common source for spyware are p2p file sharing programs and malicious websites.
San Diego based security firm Websense says that 51% of Web sites with malicious code are legitimate sites that have been compromised, rather than sites specifically commissioned by hackers.  This is especially alarming because most of the these websites are trusted and have a large volume of users.
Thankfully, there are some software programs that can scan your computer, identify and remove spyware. Some of the popular freely available antispyware software are: