When a climber uses the term "toprope," he is referring to a technique wherein an anchor is set at the top of the cliff. A rope runs from a belayer at the base of the cliff, up to the anchor and then back down to the climber. As the climber ascends the wall, the belayer takes in rope through his belay device. If the climber falls, the belayer merely locks off his device, arresting the fall. This system is designed to stop the climber's fall immediately.
The leader is the first person to climb a cliff. As the leader ascends the wall he drags a rope up that is tied to his harness. As he works his way up a wall he will put in rock protection. After the "pro" is in place, the leader may clip the rope into the gear while the follower belays from below. Should the leader fall, the follower will "catch" him in midair with the belay device.
Lead climbing may be done on both traditional and sport climbs.
The polar opposite of free climbing is aid climbing. When an individual aid climbs, he places a piece of protection and then clips a nylon ladder to it. He then climbs up the ladder and places another piece, repeating the process over and over again. The climber is using direct aid to ascend the cliff face. This is often done when it is much too difficult to free climb.