Rarely used, but is referred to when you belay someone up a route whilst stood on the ground, normally referred to as top roping
The leader is the person who climbs the climb first, taking a rope with them and placing gear as they go. In trad the person or persons who follow are seconds as hey have to remove the gear as they ascend; on single pitch sport the route is cleaned by the leader on their decent so the second person to climb is effectively just top roping (or technically speaking bottom roping).
The people who follow a leader up a climb
This happens on trad climbing, the leader sits at the top of the climb and then beelays their second(s) up a climb, from the top. What happens in a climbing wall is often referred to as top roping as well (even though it is technically bottom roping), where you've basically got a rope at a fixed point at the top, it is a generic term for climbing with a rope but when not leading.