Diffraction is the bending of a wave around objects or the spreading after passing through a gap.
It is due to any wave's ability to spread in circles or spheres in 2D or 3D space. Huygens' Principle is based on this process.
(Christiaan Huygens was unfortunate enough to be the SECOND most intelligent man of his era. He was more correct about the nature of light than Number One - Newton, but his work was ignored by comparison. He only came back into favour in the early C19th. Huygens' Principle was proved only this century. It can be used to explain reflection, refraction and diffraction.)
Diffraction processes are most noticeable when the obstruction or gap ( aperture ) is about the same size as the wavelength of the impinging wave.
It is diffraction which limits the ability of a microscope to see smaller detail. Generally a microscope can only pick details which are larger than the wavelength used, so an optical microscope cannot "see" objects smaller than about 4 x 10-7 m. That is, very approximately a micrometre in size. Viruses cannot be seen by optical microscope as they are smaller than this size.
Diffraction from various places in a gap leads to Path Differences so interference is often associated with diffraction - DIFFRACTION PATTERNS. Fresnel was the first to explore these patterns in detail though Fraunhofer preceded Fresnel by explaining a simplified model.
Optical diffraction effects can be seen with eye - in fact most of us when children have noticed it, but ignored it when becoming adults.
Diffraction plays a part in all attempts to pick up detail. It happens as light enters a lens of a camera or a telescope as the light passes around the edges of the lens. Larger diameter lenses or mirrors reduce the proportion of diffraction so reducing the diffraction "blurring". The resulting image can be enlarged far more than that of a small diameter lens.
Astronomers crave for larger and larger mirrors for this reason and they create artificially large mirrors by electronically linking telescopes. In Australia, the largest telescope is a linked set of radio telescopes covering most of Australia. The result is very fine detailed images in the radio region.
Diffraction also plays a major part in TV and radio reception ( such as mobile phones ). As telecommunications go to higher and higher frequencies ( shorter wavelengths ) such as UHF the diffraction around large hills becomes LESS so people below the hill are now in a shadow and cannot receive the signal. This forces the telecommunications people to set up more transmitters so the public can receive the product.