NM-B (Non-Metallic sheathed cable, commonly called Romex after the brand name) is the standard residential wiring method for interior dry locations. 12/2 means: 12 AWG copper conductors, 2 insulated conductors (black hot, white neutral) plus bare copper ground. Rated for 20-amp circuits at 600V. NEC requirements: 20A circuits required for kitchen countertop receptacles, bathroom receptacles, laundry receptacles, garage receptacles. 14/2 (15A) acceptable for general lighting and bedroom receptacles. Maximum conductor temperature 90°C. NOT permitted: in commercial buildings (generally), wet locations, exposed runs in garages below 8 feet (use conduit), embedded in concrete or masonry. For commercial: use MC cable (metal clad) or conduit with THHN conductors. Installation: drill holes centered in studs, use nail plates where cable is within 1.25 inches of stud edge, staple within 12 inches of each box and every 4.5 feet.