What is the difference between a Joist and Beam?

A joist can be a board as small as a 2 x 6 on up to a 2 x 12. A "beam" can be as simple as two joists nailed together. Beams will change depending on the load required of it. Some beams will require three 2 x 12s, glued and screwed or extreme loads or spans will call for engineered wood beams or even steel "I" beams with pipe columns.