Diffraction is a phenomenon where light bends or diffracts around obstacles. There are two main types of diffraction: Fresnel diffraction, which occurs over short distances, and Fraunhoffer diffraction, which occurs over long distances. Fresnel diffraction has a spherical wavefront while Fraunhoffer diffraction has a planar wavefront. The document also provides details on single slit diffraction and the mathematical expressions used to describe diffraction patterns.