I'm not sure what exact differences you're looking for, but other than the listing, I can't really think of any.

THHN is listed for temperatures not exceeding 90C in dry locations,

THWN is listed for temperatures not exceeding 75C and is also listed for use in wet locations.

Most of these types of wire insulations are dual listed as THWN/THHN. I've never personally seen just the one listing.

THHN or THWN is a thermoplastic insulation, usually PVC.

Last edited by Tom; 08/20/08 04:35 PM. Reason: poor spelling, as usual
Few things are harder to put up with than the annoyance of a good example.