A water ring vacuum pump, or liquid ring vacuum pump, utillises the water or liquid as a medium to create vacuum or compress air or gas.
The main component, the rotor equipped with blades, rotates in a cylindrical body, but is placed off-set from the centre.

See below diagram:

1:  The pump is stopped. 
2:  The pump is in rotation and the water is forced towards the cylindrical wall.
     The water will work as a piston between the blades on the rotor.

Read more details about the working priciple for a liquid ring vacuum pump.