Pedant alert! I once read (I think it was on the excellent Sheldon Brown website) that cogs are things that are driven by other cogs, and sprockets are driven by a chain- therefore what we have on our cassettes are sprockets. Ever since then I have been slightly annoyed by the use of the word 'cog' and kinda wish I never learned the difference! However, all the mtb mags refer to cassettes containing cogs, so what's going on there then, eh?
Sorry- bit of a hijack there- the talk of cogs an' that just got me thinking….