Diffraction is the bending of light as it passes the edge of an object. An example of this property is the shadow. If observed carefully, the edges of shadows are not solid, but slightly fuzzy.
So, what is going on? Diffraction can be easily explained with Huygens' Principle. Just as the front of a wave passes the edge of an object, the wavelets will cause the succeeding front to bend around the edge.
For some time, it was believed that the bending of light was not due to light itself, but the edge of the object. Not until the British Physicist, Thomas Young, conducted the double slit experiment before light was accepted as a wave.
In his experiment, he made two slits on a barrier and allowed monochromatic light (light of a single wavelength) to pass through. The result is a series of light and dark areas on the screen that could not be explained under the particle model of light. Under the wave model of light, these light and dark areas can be explained with constructive and destructive interference of waves.