Worm Food is people. That is to say, what you do in the game is eat people. You yourself are a tunelling worm-like entity with massive mandibles, and your job is to 'clear' each level, by eating all the people in it. It's 2D, but a side view, not overhead; after all, you can tunnel, quite rapidly, through the earth.
Despite apparent simplicity, the controls take a bit of practice to get used to. Left and right arrows turn your worm, but rapidly, so you mostly want to tap rather than hold them. Up is not "forward" but "accelerate" -- and at least when you are in solid earth, you can build up quite a head of steam, leaping from the ground with a roar to wreak havoc on the defenseless if tiny and pixellated human meat. You accelerate only in earth or water, however, and in the air are subject to gravity, which ultimately brings you back to plunge back into the comforting underworld.
Despite the game's relatively small window, there is at times a somewhat Sonic-like sense of speed. Levels are timed, and while the minimap shows where the meat is, you have to eat it all in the time alotted. Destroying buildings and such earns bonus points, but is not strictly required.
A few additional elements are added over time -- among other things, the meat learns how to plant mines, which you must avoid -- and Nitrome has made an effort to vary levels by design. There's not a lot to the game, ultimately, but it has a nice macabre sensibility and an interesting control scheme. When your worm goes "raaaaauuughgh," you sympathize.