Some of the available classes of documents in LaTeX are well known and widely used, such as the article and beamer classes, while others are not so well known, such as the standalone class.
I found this figure

which lists the main classes and is a good starting point, but the description is too short and still leaves one wondering when it would be more suitable to choose one class over the other and what the characteristics of each class is. Furthermore, the list is not exhaustive I think, given that I know at least one more document class that is not there (the standalone class, as I mentioned).
So my question is: what are the available classes of documents in LaTeX, and could you provide a brief description of the class and the situations where it would be recommended?
Please give only one class per answer.






standaloneclass actually simply loads a real class but uses thepreviewpackage to reduce the page size to the content. It is supposed to be used for subfiles holding only picture or similar code which are then included into a main document. Thestandaloneclass and package allow this files to be compiled standalone or as part of the main document without adjusting the file. – Martin Scharrer♦ Feb 28 '11 at 0:43