This tag is a subset of physics, and applies to the branch of classical mechanics known as Projectile Motion. It is not to be confused with bullet-physics, which is about the aptly named Bullet Physics engine.
A projectile is defined as an object that is given an initial velocity, with projectile motion corresponding to the resulting trajectory of its movement. This trajectory is often affected by factors such as gravity and air resistance, though it is not uncommon to ignore the latter in both study and practice. Projectile motion problems deal with determining things such as position in flight, trajectory, accuracy, and range.
Games of many genres deal with projectiles in motion, and game development often applies the physics of projectile motion in both artistic and mechanical endeavors. Most game development reference to projectile motion deals with arc trajectory. Use this tag when your question deals with normal physics of projectiles (as opposed to the more patterned bullet movements seen in many Shoot-Em-Up games, for example).