What Is the Difference Between Weight Machines and Free Weights?

Movements using free weights occur in a three-dimensional plane, while most weight machines allow movement only in a single plane. With machines, the movement is guided, so only the major muscles required to perform the movement are used. With free weights, the added task of balancing the weights in the three-dimensional plane recruits other functional muscles that machines do not recruit.
People new to weight lifting should probably begin with machines to train the major muscles, and then use free weights to train more specific movements.