The hopper block is possibly the most exciting new minecraft block since the piston, as it adds a new mechanic to the game. Mob grinders and farms can now be automated, as the hopper block collects items that fall into it.
You can collect items from the hopper by right clicking it. The hopper has five slots and can therefore collect five different stacks of items before it needs to be cleared.
This is still a work in progress, linked to Minecraft Snapshot 13w01b. We can expect to see the hopper optimized in the future, for the moment operation is fairly messy. Unlike other minecraft mechanic blocks, hoppers do not need to be redstone activated to work.