(From JPL's Basics of Space Flight).
Consider a system with two
large bodies, e.g. Jupiter orbiting
the sun. The third body, such as a
spacecraft or an asteroid, might
occupy any of five Lagrange points:
In line with the two large bodies are the L1, L2 and L3 points. The
leading apex of the triangle is L4; the trailing apex is L5. These last
two are also called Trojan points.
All five points are fixed points in the rotating frame.
L1, L2 and L3 are unstable fixed points, but you can stay near them
with little effort. L4 and L5 may be stable fixed points, depending
on the mass ratio of the two big bodies.