Your BB needs to match your crankset and your frame's bottom bracket threading. If you are just looking to replace your existing BB you need to buy the same type (square taper or isis or octalink), spindle length (something between 100-130mm) and threading (68mm English is a tyical designation).

If you are changing cranksets you will have to buy what ever is specified for your new crank (that also matches your frame threading). Modern cranks tend to work with bearing cups that thread on to each side of your frame and the the 'spindle' is actually part of the drive side crank that extends through and then the left crank arm attaches to it.