THHN vs THWN Wire

Almost always, yes, physically. The building wire sold today is dual-rated THHN/THWN-2, printed with both markings on the same conductor: a stranded or solid copper (or aluminum) conductor with PVC insulation and a nylon jacket, dimensionally identical either way. THHN is the dry-location, 90°C rating and THWN (specifically THWN-2) is the wet-location, 90°C rating. So the reel in your hand is usually both at once, and it takes the higher of the two ratings for the location you are in.

The letters spell it out. THHN is Thermoplastic High Heat-resistant Nylon-coated, rated 90°C but for dry and damp locations only. THWN is Thermoplastic Heat- and Water-resistant Nylon-coated, rated 75°C and approved for wet locations. THWN-2 is the water-resistant type bumped to 90°C in both wet and dry. In NEC Table 310.16 you read THHN in the 90°C column and plain THWN in the 75°C column, which is why the same #12 copper is 30 A as THHN and 25 A as THWN in their respective columns.

Usually not as the final answer, and this is the mistake that trips people up. Per NEC 110.14(C), the ampacity is limited by the temperature rating of the terminals, which on breakers, lugs, and devices is almost always 60°C or 75°C, not 90°C. You use the 90°C column only as the starting point for temperature-correction and conductor-bundling adjustments, then the terminal rating governs the final ampacity. So THHN's higher number is derating headroom for hot or crowded conduit, not a license to load the conductor to 90°C.

Not on its THHN rating alone. In a wet location, in underground conduit, or anywhere water can enter the raceway, you need the wet-location rating, which is THWN or THWN-2. Because modern wire is dual-rated THHN/THWN-2, the exact same reel is fine wet, but you are relying on the THWN-2 marking to make it legal, not the THHN marking. If a conductor is marked THHN only, keep it dry.