Swash Plate Pump or axial piston pump 

This video gives a basic understanding of working of swash plate pumps. These pumps are commonly used in steering gear system.
Swash plate pumps have a rotating cylinder containing pistons. A spring pushes the pistons against a stationary swash plate, which sits at an angle to the cylinder.The pistons suck in fluid during half a revolution and push fluid out during the other half.
For a given speed swash plate pumps can be of fixed displacement, or variable by having a variable swash plate angle. The greater the slant the further the pistons move and the more fluid they transfer.