Answer:
The machines that use wave energy fit mostly in two categories; one which resembles a buoy and the other a sideways door with its hinges being on the sea-bed.
The buoys are vertically oscillating columns that float on the surface of the water. The regular oscillation of the ocean tides causes the buoys to bob up-and-down which drive a piston which turns an electrical generator producing electricity.
The other things can be viewed as a door hinged to the sea bed, and swings with the back-and-forth movement of ocean water. The oscillations again drive a piston which powers the generator.
Examples of each:
AquaBuOY - http://www.finavera.com/en/wavetech
WaveRoller - http://www.aw-energy.com/