Dan's comp magazine has a great little write-up in the corner of the crank page.
Here is the info minus the pics:
American- The original BB for BMX also referred to as Pro size. The bearing rests in a cup that is pressed into the frame. The bearings are large and durable but typically heavier than other types.
European- European BB's use small cups that thread into the frame, making installation easy. Since the bearings are small, most Euro BB kits use four bearings to spread the load.
Spanish- Spanish BB's use a bearing that presses directly into the frame without the use of a cup for a clean, flush look. Spanish bearings are larger than the European bearings, but still smaller than an American bearing.
Mid- Mid BB's use an American sized bearing that presses directly into the frame without the use of a cup, leaving a flush fit. The bearings are strong, but slightly heavier than Spanish bearings.
Personally, I've had some creaking issues with a Euro BB that required lots of grease to remedy.