If a player passes the ball back with any part of his body except his feet, the goalkeeper can pick it up. An unintentional pass or touch is not considered an offense.
If a goalkeeper does pick up the ball after a deliberate back pass with feet or a throw-in, an indirect free-kick will be awarded to the opposition on the spot where the goalkeeper touched the ball with his hands. The offense is the goalkeeper touching the ball with his hands, not the actual back-pass.
The rule was brought in 1992 to discourage time-wasting and other overly defensive tactics.