The chainset (in the UK) or crankset (in the US), is the component of a bicycle drivetrain that converts the reciprocating motion of the rider's legs into rotational motion used to drive the chain, which in turn drives the rear wheel. There are currently four chainset/bottom bracket interfaces that you're likely to come across. If your bottom bracket is made by Shimano and it has splines, then you need an Octalink chainset. If your b/b is NOT made by Shimano and it has splines, it's almost certainly ISIS. There are a very few non-Shimano chainsets with Shimano Octalink interfaces so, to be sure, count the splines - ISIS has 9, Octalink has (surprise, surprise) 8. No matter which brand of b/b you have, if it has a square interface, it will require a traditional square taper chainset. If you have one of the new Hollowtech II systems, you will need to choose the exact chainset that matches your bottom bracket. Most MTB chainsets are now 9-speed compatible but these will work with an 8-speed system.