I've been conditioned over time to think of a .org address as signalling that the site is to do with some kind of non-for-profit kind of organisation. Not that there seems to be a rule that this must be so. .com more commonly represents some kind for for-profit undertaking, though again, it's not a rule. .net is often a little more undefined and open to anything. As I said, these aren't rules as such, but I find them useful as a guide to what to choose.