Bottom Bracket Summary
The bottom bracket on a bicycle contains an axle to which the crankset is attached and the bearings that allow the axle and cranks to rotate. (The chainrings and pedals are attached to the cranks.) The bottom bracket fits inside the bottom bracket shell, which is part of the bicycle frame.

There are currently a lot of different bottom bracket formats and, of course, there are compatibility issues between all of them. It's always wise to know what types of bottom brackets your frame is made to accept and to know the type of bottom bracket your crankset is made to interface with.

The key features needed to be considered when picking between bottom brackets are compatibility, stiffness, durability and resistance (less is obviously better).
Full Article