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1.Two separate wave fronts originating from two coherent sources produce interference. Secondary wavelets originating from different parts of the same wave front constitute diffraction. Thus the two are entirely different in nature. 

2.The region of minimum intensity is perfectly dark in interference.In diffraction they are not perfectly dark. 

3.Width of the fringes is equal in interference. In diffraction they are never equal. 

4.The intensity of all positions of maxima are of the same intensity in interference. In diffraction they do vary.

A classic application is in diffraction gratings that break white light into its spectral colors. Another application is in holography, a way of reconstructing three-dimensional images of objects with laser light.Many optical coatings depend on optical interference for their optical properties, including the anti-reflection coatings on most complicated lenses.