The ulna and the radius are the two bones in the forearm.
Latin: Ulnaris et. radialis.
If you hold your arm at your side with the palm forward ulna is palpable on the medial (inside) part of the forearm, whilst the radius is palpable on lateral (outside) side.
In layman terms, the radius is on the side of your thumb, and the ulna on the side of the pinky-finger.