Leading or lead climbing (in French: aller en tête) is a belay system where the climber "leads" the rope and his partner. That means there's no rope coming from above him (as in top-rope. As he climbs, the climber clips the rope into runners, attached to anchors (bolts on sport routes, other nuts on trad routes etc. Therope also passes through a belay device and held by the belayer. In case of a lead fall, the belayer arrests the fall. In theory, he leader falls (about) twice the distance he was from the last piece he's clipped into.
The belayer should always be ready to arrest asudden fall, he should always have enough slack in the system so that he does not pull his leader off the route. On the other hand, there should be just enough slack and not more, so that a fall, if it occurs, would not be longer than necessary.
Leading is considered (and indeed is) more chalenging, both mentally and physically than toproping. If this is so on sport routes, with safe, pre-set bolts, It is even more accentuated on tread routes, where the leader has to place his pieces, choose the right length runners and clip into them. This is why many climbers claim that leading is the "real" climbing. An experienced leader can climb anywhere, without the constrictions of length, or the need to walk around to set a rope in advance (as in top-rope).
Contributions to this page were made by Mica Yaniv and others...