What makes a carbine a carbine?
http://en.wikipedia.org/wiki/Carbine
Basically, a carbine is a shorter, handier version of a full size rifle.
With today’s technology and quality, it is possible for a carbine to be virtually (I think) as effective as a full size rifle, so it is much more common to see carbines in use.
(your mileage may vary)
Kommodor summed it nicely. I would also add that during WW2 the M1 carbine was issued as a substitute or in addition to the 1911 pistol, to be carried by officers, cooks etc. Lighter and smaller than the main battle rifle but easier to become proficient with and more accurate at longer ranges than the pistol.