Interference is interaction between two specific frequencies whereas diffraction is interaction between a frequency and a space of equal size to its wavelength to produce a new frequency of the same wavelength.
Interference is the result of the primary signal interacting with a secondary signal of equal wavelength canceling out part or all of the primary signal. Such as the way two singers off-key will sound as though they warble as their voices move into and out of the same notes whereas two singers on key will sound as though they share a single voice.
Diffraction is the signal being deflected off course while passing through a space or slit in a material object as when a radio signal breaks up when passing through a tunnel.
I might always be wrong.