By default, 10 is admin (on networks that support it) and can only be added by a founder or admin, I believe... perhaps just a founder. 5-9 is auto-op, 3-4 is auto-voice, 0-2 is really nothing, and <0 prevents opping.
These can be changed, of course, and may also have different defaults on different networks. Basically, though, only at the certain levels (3,5,10) is there any difference in what happens to the user. 5 and 6 do the same thing, for example.