Grenade
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 incediary effect, illumination, and tear gas.
Hand grenade
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.
Most grenades have a short time fuze, activated when a safety ring is removed and a spring-loaded handle is allowed to fly off, which strikes a spark and ignites the fuse. Typical delays are 3-7 seconds after the handle springs free.
Historic
World War I
World War II
Modern
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.
Grenade launchers
Rifle grenade
Dedicated grenade launchers; individual weapons
The U.S. and many other countries use variants of 40mm grenades, fired from:
M79
A weapon 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 made up of a bundle of darts for short-range antipersonel work. The other grenades must fly a minimum distance before they arm.
M203
The single-shot launcher attachment for M16 (rifle) or M4 (rifle); they mount under the barrel and fire the same 40mm grenade as the M79
Dedicated grenade launchers; crew-served or aircraft
Mark 19
A 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.