There are ten different eye layouts—indeed every technological method of capturing an optical image commonly used by human beings, with the exceptions of zoom and Fresnel lenses, occur in nature. Eye types can be categorised into "simple eyes", with one concave photoreceptive surface, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface. Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution—the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures. Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomeric. These two groups are not monophyletic; the cnidaria also possess cilliated cells,  and some gastropods, as well as some annelids possess both.

Some organisms have photosensitive cells that do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. These are not considered eyes because they lack enough structure to be considered an organ, and do not produce an image.
Given this paragraph about different types of eyes, how are eye types categorized and why?
Eyes are categorized into two types, "simple eyes" and "compound eyes". In this case, "simple eyes" do not mean that they have reduced complexity or acuity, but rather that they have one concave photoreceptive surface. This is in contrast to "compound eyes" which are comprised of a number of individual lenses on a convex surface. In other words, eyes fall into two groups based on their photoreceptor's cellular construction either being cilliated (as in the vertebrates) or rhabdomeric.