When waves hit the coast they force water and air into cracks in the rocks. As the water surges it compresses the pockets of air and when the wave retreats the air is released and quickly expands. This continued process weakens the joints and cracks eventaully causing the rock to shatter.
Abrasion (also called corrasion )
Breaking waves throw sand, pebbles and sometimes boulders against the cliff face. These scrape away at the rock rather like sandpaper, eventually causing undercutting at the base of the cliff.
Rocks and boulders eroded from the coast collide with each other and the rock face in the breaking waves. They become smaller and more rounded.
Sea water dissolves soluble minerals and material from the rock. In particular limestone and chalk coasts are very susceptible to this type of erosion.
This video explains how the processes of Coastal Erosion work