Reflection occurs when a wave encounters a steep obstacle, such as a large rock or a sea wall. When the wave encounters the obstacle, most of its energy bounces back, or is reflected. If the water is deep, the wave does not break, but just changes direction. You can observe this when looking over a steep cliff facing the open ocean. When a wave strikes a sea wall, much of the energy is also reflected back to sea.
Diffraction occurs when wave energy spreads out through an opening such the mouth of a harbor.
As the wave moves through the opening it is confined. But when it passes through the opening and is no longer confined it begins to spread out or diffract. Refraction is a change in direction of a wave because of a change in propagation speed. This occurs because the wave slows down as it moves into shallow water. Since one portion of the wave is moving at one speed, and another is moving at a different speed, the wave crest appears to bend.