A sea wall is a wall made out of a hard material like concrete that reflects waves back to sea.
The benefits of the sea wall are that it prevents erosion of the coast. It also acts as a barrier to prevent flooding.
However, the drawbacks of the sea wall are that it creates a strong backwash, which erodes under the wall. Sea walls are also very expensive to build and to maintain.
Rock Armour is when boulders are piled up along the coast
The benefits of rock armour are that the boulders absorb wave energy and so reduce erosoin and flooding. It's a fairly cheap defence strategy.
However, the drawbacks of rock armour are that boulders can be moved around by strong waves, so they need to be replaced.
Groynes are wooden or stone fences that are built at right angles to the coast. They trap material transported by longshore drift.
The benefits of using groynes are that groynes create wider beaches which slow the waves. This gives greater protection from flooding and erosion. They're a fairly cheap defence strategy.
However, the drawbacks of using groynes are that they starve beaches further down the coast of sand, making them them narrower. Narrower beaches don't protect the coast as well, leading to greater erosion and floods.