The bottom bracket is the interface between the bike frame and the crankset. As such is has to match both.

Frame issues include the BB shell width, diameter, thread pitch and direction. For practical purposes, diameter, thread pitch and direction can be summed up by saying English, Italian or French. Don't assume that a bike frame with an Italian sounding name will necessarily take an Italian bottom bracket.

Crankset issues are spindle length and square taper, splined or exo-bearing interface. All of these issues ar typically specified by the crank manufacturer. Sometimes the specified spindle length will be an option (113/118). If that's the case choose the 113 spindle length if your rear dropouts measure 130 mm, and the longer, 118 version for 135 mm dropouts.

Oh - and if you think your question was stupid you should see what has happened to some of the people who didn't think to ask it.