This is the spreading out of waves when they pass an obstacle or through a gap. Diffraction occurs the most when the gap is of similar size to the wavelength of the wave. Diffraction can occur with any wave: water waves, light, sound, radio waves etc.
The diffraction of X-rays can be used to study the structure of crystals. This is because the wavelength of the X-rays is similar in size to the 'gaps' between the crystal’s molecules.
When parallel waves approach a gap in an obstacle, they are diffracted through the gap. A great example of this is shown in this picture. The waves are coming from the bottom right of the image and notice how they are diffracted through the gap between the islands. extracted from : http://www.suntrek.org/factary/d.shtml
