For this installation, we are using 12-2 NM electrical cable. The 12 refers to the gauge, or thickness, of the individual wires, or conductors. The 2 indicates there are two insulated conductors (a bare or green-jacketed ground wire is also included). The NM stands for non-metallic, or plastic-jacketed, cable.
Pull the cable through the drilled holes, working from one end of the circuit to the other. When wiring a series of lights, run a single length of cable from one fixture location to the next, but leave plenty of excess wire to work with at each fixture.
Follow the "Rule of 8's" -- leave at least 8" of wire extending out past each junction box, use wire staples to attach the cable to the wood framing within 8" from the box, and fasten the cable within 8" from where it extends into the wall.