Diffraction is the phenomenon exhibited by light waves, sound waves, electromagentic waves, waver waves, matter waves. When waves encounter objects of the size of their wavelength, then they bend round the corners or edges of the obstacles. Thus waves enter the regions which we expect to be shadow regions as in case of rectilinear motion of waves. Also light waves seem to spread out from very narrow slits in an opaque medium.
These effects are also called diffraction.
When two or more waves combine physically (overlap a region), then the displacements in time are algebraically added to obtain the resultant wave form. If the waves are constructively superposed, we get a bigger waveform. If they are out of phase by 180 deg., then they destruct each other. This phenomenon is called Interference of waves. All waves exhibit interference.
In diffraction, waves that originate from the same source, interfere with each other. They have phase differences coming from varying path lengths. Diffraction patterns are formed from interference of many secondary waves. Interference is between any two waves. The result of interference can be time varying or location varying or it can be fixed. In diffraction the pattern is fixed in space and time.