This is a touchdown scored after a pass has been caught and the receiver makes it into the end-zone. The ball does not have to be caught in the end-zone in order to be considered a receiving touchdown, only that the play involved a pass and resulted in a touchdown. When a receiving touchdown is awarded, both the quarterback (or whomever threw the ball) and the receiver receive credit for the touchdown.
In the modern game, receiving touchdowns are most common than rushing touchdowns. This is mostly a result of the increased skill level of quarterbacks and receivers, and the increased speed of all players involved. Furthermore, the NFL has shaped the game via rules and regulations so that passing plays are more common. It should be noted that a receiving touchdown requires that two individuals be credited with a touchdown. This phrase does not apply to an interception returned for a touchdown, which is referred to as a pick-six.