Lagrange Points are positions in space where the gravitational forces of two celestial bodies produces an enhanced amount of attraction & repulsion. This causes a spacecraft or a satellite present at that point to remain in a fixed position without the need of control thrusters or orbital correction with minimal fuel consumption. 

L1 Lagrange Point

L1 Lagrange point is the point between the earth & sun where it remains in a constant position between them. The L1 point is useful to observe or monitor the sun, as the constant stream of particles from the Sun & the solar winds reaches L1 before it reaches the earth. Satellites such as SOHO & the ESA/NASA solar watchdog are positioned here.

L2 Lagrange Point

The L2 Lagrange point is located 1.5 million km behind the earth as viewed from the sun. The L2 point is primarily used to observe the vast oblivion behind the Earth for astronomy. Example - Herschel, Planck & Gaia.

L3 Lagrange Point

L3 Lagrange point lies behind the sun, and it is a point that cannot be observed from earth. It offers the potential to observe the far side of the earth. However, no satellites have been placed at this point because feasible communication with a satellite is difficult to establish from the earth due to the Lagrange point being placed behind the sun.

L4 & L5 Lagrange Points

The L4 & L5 Points lie at 60° ahead of and behind the earth. L4 & L5 points are the most stable Lagrange points and primarily dust & asteroids tend to accumulate in this region. Satellites or spacecraft at L4 & L5 points are extremely stable at this point.