All Mountain Bike Parts Descriptions

#13) Bottom Brackets (BB)- The Bearings for your crank.
  Available in "loose cup" or "cartridge". Loose cup BB's can be repacked with grease, but axles and cups are usually made of softer materials so they wear out sooner. The Cartridge type can't be serviced but, because the bearings are sealed away from dirt and water, they last for years, and are usually made with higher quality cartridge bearings. The latest BB's on the market place have the cartridge bearings out, and beside the frames BB-shell. This allows for larger axles which are thinner and lighter to be used and has a larger surface area for the Crank to attach to.  
  Bottom brackets are threaded into the frames BB-shell with a right handed thread on the left side and a left handed thread on the right side. This seems silly, but it was probably designed that way, long ago when a loose cup started to tighten itself into its bearings, slowing the rider down. He decided it would be better to have the BB cups loosen off while riding rather the having the bearings crushed, and I agree, hence the wacky way the threads go.
The axles used in BB's come in square, Octalink, ISIS, and various BMX splines, to match the cranks. There are two standard frame BB-shell diameters BMX or road, and three widths 68, 70 and 73mm. A 70mm is standard on road bikes, but newer outer mount styles are even larger.

Bike Parts Diagram