Historically, a grenade was a small explosive charge that could be thrown by hand. The hand grenade still exists but has evolved, but there are also a number of grenades fired by specialized launchers.
The most common filler for a grenade is explosive, sometimes broken into "offensive grenades" that do not scatter metal fragments, and the more common "defensive grenade" (or grenade without qualifier) that has a blast and fragmentation effect. Other fillers include colored smoke for signaling, white phosphorus for incendiary and screening smoke effect, thermite for incendiary effect, illumination, and tear gas.
Once past the early stages, there was considerable differentiation of grenade types and designs among nations. For example, the U.S. Army's general-purpose grenades increasingly were shaped more and more like a baseball, assuming that many soldiers could throw that shape accurately. Countries that do not have a sport like baseball often used a "potato masher" type with a cylindrical explosive charge with a concentric, throwing handle, which could get more distance than a ball-shaped grenade.
The fuze of a typical grenade consists of a safety ring, a spring-loaded "spoon", a fuse igniter, and a fuse that leads to the detonator. When the safety ring is pulled, the spoon is free to move, although the user can delay the triggering by holding it down -- this is also common in improvised explosive devices/boobytraps, where the trip wire or other activating mechanism releases pressure on the spoon.
When the spoon is freed, its spring drives an igniter, much like an ordinary match -- creating fire by friction on a sensitized chemical mixture. The flame moves to a pyrotechnic fuse, which gives the delay before explosion, when the flame front reaches the primary explosive detonator and the detonator sets off the main charge. Especially in Soviet-bloc grenades, the fuse may be interchangeable to provide different delays -- effectively zero for improvised explosive devices/boobytraps, 3-5 seconds for general infantry use, 7-12 seconds for long range.
Some grenades, such as thermite used principally to destroy equipment, do not have a time delay as they are not thrown.
World War I
World War II
In addition to grenades intended for combat, the "flash-bang" is used for police hostage rescues, military special operations, etc. It disorients with an extremely bright light and loud noise, but rarely does more damage than a broken eardrum. The user can wear eye and ear protection.
Grenades can be mechanically launched as well as hand-thrown. One characteristic of a grenade launcher is that it propels the grenade at speeds considerably lower than that of a bullet. In some respects, this is an advantage, as it offers the opportunity to develop large, slow, nonlethal grenades, which act by kinetic energy of the entire round (i.e., a "beanbag") or of submunitions such as rubber balls.
Essentially obsolete by mid-1950, rifle grenades are launched from standard infantry rifles, using a blank cartridge as a gas generator, and a grenade launcher that attaches to the muzzle of the rifle.
Dedicated grenade launchers; individual weapons
The U.S. and many other countries use variants of 40mm grenades, fired from:
The M79 grenade launcher is carried much like a large shotgun or tear gas gun, the M79 breaks open at the breech to accept a single grenade. In addition to blast/fragmentation, smoke, white phosphorus, and illumination, it accepts a flechette round made up of a bundle of darts for short-range antipersonel work. The other grenades must fly a minimum distance before they arm.
A major disadvantage of the M79 is that its user either must also carry a rifle, or be restricted to a specialized single-shot weapon. The M203 grenade launcher is a single-shot launcher attachment for the M16 rifle or M4 carbine; they mount under the barrel and fire the same 40mm grenade as the M79.
There were disadvantages to the M203, including its single-shot limitation, the need for a specific mount, and a limitation on grenade dimensions. The newer M320 grenade launcher mounts to a multiple-weapon Picatinny rail, loads from the side rather than the bottom allowing for other grenade lengths, and can be adapted to be an independently carried weapon.
Dedicated grenade launchers; crew-served or aircraft
The MK 19 40mm automatic grenade launcher is s belt-fed full-automatic automatic grenade launcher, with versions that can be fired from a tripod, from a mount on a HMMWV or other vehicle, on river patrol boats, and helicopters.
Rocket propelled grenade
A hybrid weapon, generally called a rocket propelled grenade, is typified by the German Panzerfaust or Soviet RPG-7. These have a large warhead, really too large to be thrown by hand, that protrudes from the muzzle of a rocket launcher. The grenade is similar to early anti-tank grenades, in that it is a Munroe-effect shaped charge optimized for armor penetration. They have a range of a few hundred meters.
These weapons are inexpensive and extremely common. They can also be used against ground targets and, with some modifications, helicopters.