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...you should measure the shell width on the bike in front of you with either a metric straight edge or a caliper, to make certain the shell is, indeed 68mm wide. If the crank is Octalink (you can Google a picture if you're not certain), than that's pretty much all you need to know. They are all English standard threading now. Anything will work, but the available alternatives vary widely in quality and construction. I generally spend a little more, and try to get something made with higher QC standards.
Note there are 2 versions of Octalink, and the spindle lengths can vary.
The Octalink system uses a spindle with eight splines. The splines provide a contact area between crank and spindle for an interface. Octalink exists in the marketplace in two variants, Octalink v1, and Octalink v2. The difference between the two can be seen by the depth of mounting grooves on the bottom bracket spindle. V1 spline grooves are 5 mm long, while V2 grooves are 9mm long. Shimano 105, Ultegra 6500 and Dura Ace 7700 cranksets mate to version one bottom brackets, while later mountain bike designs use the deeper-grooved version two. The system is proprietary and protected by Shimano patents and license fees.
There's a special tool you use to pull the cups that fix the BB unit in place.