It is also called as on Oil Damper, and the damper combined with coil spring is typically used as vibration-proof device(shock absorber) for automobiles, and has tub type and piston type.
General structure of the damper is same as the gas spring, but it has a piston with small hole(Orifice) of the valve inside the cylinder filled with oil instead of gas.

If the piston inside the cylinder moves, oil is transferred from a space to the other space inside the cylinder. As the oil passage will be narrow by the hole of valve, the oil pressure at the side of narrow space increases, and thus the resistivity(damping force) will occur.
This resistivity has nature of increasing in proportion to squaring of relative speed between the cylinder and the piston.