What makes a rifle a carbine rifle?

Answer 1

A carbine is generally a rifle with a shorter barrel to make it easy to carry or to use indoors or from inside vehicles. (Example: M16 rifle; M4 carbine. Same basic weapon except one barrel is several inches shorter.

Sometimes a "carbine" will also mean a weapon that looks like a rifle but shoots pistol-caliber ammunition or other weaker ammo. Example: M1 Garand .30 rifle; M1 .30 carbine (the carbine is several inches shorter, a few pounds lighter, and fires a much smaller weaker round, intended for close-range use only).