These are all properties of waves and their definitions are useful to remember for exams. It can also be helpful to draw diagrams of each to help you to remember the difference. On the diagram it is useful to mark on a line for 'normal', this is a straight line that is perpendicular (makes a 90 degree angle) with a surface.

Reflection: This is when a wave hits a surface (e.g light hitting a mirror) and is bounced back. Remember that in reflection the angle of incidence (this is the angle that the initial wave makes from the normal), is the same as the angle of reflection (the angle that the reflected wave makes from the normal).

Refraction: When light travels from air into another material (such as glass) at any angle other than normal (perpendicular), it slows down. As it slows down it changes direction, meaning that the angle that it is travelling at changes - the angle becomes closer to the normal. (a picture would help to explain this)

Diffraction: Waves spread out as they enter an aperture (gap). The most amount of diffraction happens when the wavelength is a similar size to the gap.