What is the difference between romex and thhn wire?

Romex is a brand name for non-metallic (NM) sheathed cable, containing multiple THHN (Thermoplastic High Heat Nylon) insulated conductors, a bare ground, and a paper wrap, all encased in a PVC jacket. THHN is an individual conductor type, typically used within conduit or other protective raceways, not as a standalone cable like Romex.