A bullet is a projectile propelled by a firearm, sling, or air gun. Bullets do not normally contain explosives, but damage the intended target by impact and penetration.
The word “bullet” is sometimes colloquially used to refer to ammunition in general, or to a cartridge, which is a combination of the bullet, case/shell, powder, and primer.
This use of ‘bullet’, when ‘cartridge’ is intended, leads to confusion when the components of a cartridge are discussed or intended. See the reference section for more detail.