As mentioned above there are two ways used for push back by using a tow bar or tow braless pish trucks.

There is a third way but its not that common and rarely user it is the (Power Push Unit).
Unlike the standard pushback process that uses a pushback truck and a tow bar attached to the nose wheel, the PPU is a remote controlled truck directly attached to one of the main landing gear. It pushes the aircraft backwards and the aircraft pilot is in charge of steering the aircraft with the nose wheel, which can move freely. So basically the pilot has to allign him self by using the nose gear.