As Tom listed, there is several differnces. What is important is what what you application is. Is the environment wet (THWN) or damp (THHN) and ambient temperture rating needed. A big one would be coduit fill. Given that THWN is rated at 75 degrees, wire derating becomes a bigger issue sooner then THHN. I typically use only THHN and XHHW. These two cover pretty much most typical applications so stocking is less of an issue at the warehouse and service rig. Much if not all the THHN I have used is THWN rated as well so either way, I am covered. I never specifically needed THWN so I have not really look at THWN in detail.
I had this discussion with a co-worker recently. I think that the conductors in Romex are truly rated as THHN, while the spools of "THHN/THWN" are really type THWN. If you compare the two, THWN has a thicker clear nylon covering than the conductors in Romex cable.
My theory is only a guess though. I think that manufacturers sell it as dual-rated to save the expense of making two types of wire, not to mention the cost for contractors to stock both types. Unless there was a significant cost difference, like back in the TW/THW days, then it might be worth making and stocking both types.
On a side note, does anybody remember opening a fresh box of TW and getting a sniff of it? Man, I used to love that smell.
The stuff I saw was red, black and green twisted and I am sure it said THWN. My neighbor was using it on his dock (old irrigation man) and was trying to pull it in 3/4" RNC. I gave him a bottle of yellow 77 to get it to go.
Thanks to all for you responses. I am running through 1 1/4" conduit to a boat dock sub panel. The boat takes a 30 amp double and there will be an additional 15 amp single for a recepticle and one other 15 amp single for a switched water pump. I am coming out of the main with a 50 amp. the sub panel will be mounted on the dock post. I hope this helps explain why I am asking. Thanks again.
The wire is simply dual rated. They made a waterproof wire that can stand 90c. Just bear in mind, when you use THHN/THWN in a wet location you have to follow all the THWN rules, particularly the 75c ampacity rule. ... No I can't explain that.