From HomoExcelsior

The Lagrange (sometimes called Libration) points are positions of equilibrium for a body in a two-body system. The points L1, L2, and L3 lie on a straight line throught the other two bodies and are points of unstable equilibrium. That is, a small perturbation will cause the third body to drift away. The L4 and L5 points are at the third vertex of an equilateral triangle formed with the other two bodies; they are points of stable equilibrium.

To the right is a diagram of the Earth/Moon two body system's lagrange points.

---