The player's handbook describes the process for determining ability scores via 4d6 drop the lowest (or, actually now that I think about it, it might just be 3d6 and the 4d6 drop might be another variant rule). But I have very rarely actually used that, and instead used the 'point buy' system, starting from 8 and 'buying' points up to a maximum of 18.
I can't find a source for that in the PHB, the DMG or the SRD. Where did it come from? A 3.0 book? A 2e book?