Grenades are small devices that contain explosives, and are launched or thrown so that they explode away from the person who threw them.
Hand grenades are thrown by hand. Other grenades have launchers. Some launchers attach to rifles, others are carried by themselves, such as rocket propelled grenades, called RPGs.
Explosive grenades often have steel cases that are cut so that bits of steel will act like bullets when the device explodes. These are called fragmentation grenades. Others merely explode, and are called concussion grenades.
Grenades use several chemical reactions to work. When the grenade leaves the thrower’s hand, a spring pushes a pin down on a percussion cap. That cap explodes, and lights a four second fuse. That fuse eventually sets off a detonator, which then sets off the main explosive charge.
Not all grenades are actually explosive. Some are designed to set fires. These are called incendiary grenades. Others release smoke, or tear gas, or just make a loud noise and a flash of light to stun people.