The minecart loader is an advanced form of the hopper, only outputting downwards, but which is capable of quickly loading and unloading minecarts. It will "catch" a rolling minecart that passes above or below it, then proceed to load (or unload) it until either the source inventory is empty or the destination inventory is full, finally giving the minecart a push in the same direction it had been traveling, allowing the cart to continue on its way. This block makes it easier to fully load or unload a cart without the need for complicated redstone setups with the vanilla hopper, detector rail, comparator, and a secondary "bumper" cart.
The loader can handle multiple carts at one time, although it has the same loading/unloading limitations as a vanilla hopper with regards to speed.
