The squid has an unusual way of movement. It can suck water into its body cavity and then eject the stored water from special organs at a very fast speed. It is also able to accurately control the direction of the jets of water, which allow it to steer.
The movement of the squid is explained by Newton's Third Law which states that for every action there is an equal and opposite reaction. When the water is being expelled as a jet from the squid's body, it exerts a force on the squid's body that moves it in the opposite direction.
If the mass of the water being expelled is Mw, the velocity with which it is expelled is Vw and the squid's mass is Ms, the velocity with which its body is pushed in the opposite direction is given by Vs = Vw*Mw / Ms.