Bento are basically lunches in little boxes. The most usual form is a double-decker lunchbox with a lid, but they can get pretty fancy. (Caito sent me one that's a bento box, a little cup, an insulated water bottle holder, and a container for a little washrag you can use to clean your hands with. It's awesome.)

Cooking cute is totally aces at explaining it better:

http://www.cookingcute.com/aboutbento.htm

Basically bento is the new food porn in America. :)