Every conductor and cable carries printed markings along its length, and those letters describe where the wire is allowed to be installed and how hot it is permitted to run. Reading them takes a minute and prevents a category of mistake that is expensive to correct later.
THHN is a single insulated conductor, thermoplastic insulation with a nylon jacket, rated for dry locations at ninety degrees Celsius. It is what gets pulled through conduit. Most of what is sold is dual-rated THHN/THWN, and the W in the second rating means it is also approved for wet locations, which matters because the inside of a buried or outdoor conduit is legally a wet location even when it looks dry.
NM-B is the flat sheathed cable that most houses are wired with. The N and M stand for non-metallic, referring to the outer jacket. The B indicates the individual conductors inside are rated at ninety degrees, though the cable as a whole must be used at sixty degree ampacities. It is for dry, protected, indoor locations only, which rules out burial, exterior walls exposed to weather and anywhere it could sit in water.
UF is the one that looks similar and behaves differently. It is moulded rather than jacketed, with the conductors embedded in a solid grey compound, and it is rated for direct burial and sunlight exposure. It is noticeably harder to strip for exactly that reason.
The jacket also carries the conductor size and count. A marking reading 12/2 with ground means two twelve-gauge current-carrying conductors plus a bare or green grounding conductor. 12/3 adds a red conductor, which is what three-way switch legs and shared-neutral circuits use.
Colour helps as a shorthand in newer cable, where fourteen-gauge is white, twelve is yellow and ten is orange, but that convention is a manufacturing standard rather than a code rule and older cable predates it entirely. The printed marking is what counts.
Older houses complicate this because cable predating current standards may carry faded markings or none that are still legible. In that situation the conductor size has to be measured rather than read, and the insulation condition assessed on sight, which is one of the reasons rewiring estimates on very old properties come with more caveats than newer ones.
Further reading: Electrician Wichita Pros
