see the glossary here http://www.bbc.co.uk/education/guides/zdwnb9q/revision
reflection: There is a reflection when waves bounce off a surface.
refraction: Process by which a wave changes in speed and sometimes direction upon entering a denser or less dense medium. For example, a light ray changes direction when refracted by a lens.
diffraction: The spreading out of waves when they pass through a gap or around an obstacle.
reflection also has a slightly different meaning in maths where you might transform a graph into it's mirror image.