Well with after market wheels they're either 'direct drill' and the centre of the wheel is a set size for a certain manufacturer . . . or in most cases the centre bore is massive and a adapter (called a spigot ring) is needed to set the centre bore to match that of your car.
The idea is to make sure the wheel is centred and spins in a 'Hub Centric' fashion (i think that's the right term?!?), otherwise it'll spin erratically and i.e.. up and down and wobble like crazy!!
Hope that makes sense . . .