A projectile is a type of weapon that is propelled towards its target. If you fire a cannon, the cannonball is a projectile, but the cannon itself is not.
When projectile refers to a weapon, it is always propelled by something else. Think of the bullet in a gun or a spear thrown by a person. A rocket, which is self-propelled, is also called a projectile. As an adjective, projectile means "propelled forward" as in a projectile force or weapon. Sometimes you see the expression, projectile vomiting, a particularly violent way to throw up.