SPANISH (SPAN) Spanish Bottom Brackets 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 the American bearings. 1-7/16" (36.5mm).
MID Mid Bottom Brackets 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. 1-5/8" (41.2mm).
AMERICAN (AMER) The original Bottom Bracket for BMX, also referred to as a 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. 2" (50.8mm).
EUROPEAN (EURO) European Bottom Brackets use small cups that thread into the frame, making installation real easy. Since the bearings are small, most European Bottom Bracket kits use four bearings to spread the load.
Note that when your frame has a 73mm bottom bracket shell, you will need a slightly longer tube spacer (53mm instead of 48mm) than what would typically come with a Spanish bottom bracket. Spanish bottom brackets are typically for bmx bikes, and the vast majority of bmx frames have 68mm wide bb shells. So, many bmx bottom brackets only come with one tube spacer, and it's the one for 68. I have done a couple of bmx frame builds on bmx bikes that did have 73mm bb shells, so I went down to my local bike shop and picked up the 53mm tube spacer separately.
Also, many MTB frames require a 6 inch spindle in order to get your crank arms wide enough to clear the frame's chainstays. For my Black Market Mob for example, I had to get the 6 inch spindle option from Profile to go with Profile bmx cranks. Some DJ frames, though, they know you're going to run bmx style cranks, so the chainstays are set a little narrower, and you do get clearance with a typical 5.75" bmx spindle.