Why do we see the same side of the Moon?

The Moon revolves around the Earth once every 27 days. But the images of the back side of the moon were obtained only after satellites were launched to capture the same. This is because of a phenomenon referred to as tidal locking. 

The moon rotates along its own axis just like Earth. This too takes approximately the same time. This is not a coincidence. At one point of time in the distant past, the moon’s rotational speed was higher than what it is presently. However the gravitational force the Earth exerts on the Moon, similar to the gravitational force the Moon exerts on the Earth for the production of tidal waves, acted as a correcting measure that not only reduced the speed  but ensured that one phase of the moon was always exerted towards Earth with a greater force than the other side. The force needs to be imagined as torque which has an effect on the angular momentum of the object.

This is not a unique phenomenon which is observed just between Earth and Moon but rather across different planets like Jupiter, Uranus and Plato and the satellites that co-orbit them.