{document-classes} is for questions relating to the document class of the TeX file. This sets up various document-wide formatting elements.
document-classes is for questions relating to the document class of the .tex file. This sets up various document-wide formatting elements. Questions with this tag relate to choice of document class.
Some popular document classes include the standard article, book and report classes. KOMA script offers replacements for those standard classes: scrartcl, scrbook and scrrprt. To find out about differences between these classes, see Regarding the `book`, `report`, and `article` document classes: what are the main differences?. The memoir class is another popular choice.
Other, less common classes used for special purposes include the letter class and its KOMA replacement scrlttr2, the standalone class for testing TikZ pictures, for example. There is minimal which is another class used mainly for testing, but not recommended for minimal working examples (MWEs) on here. For typesetting CVs, there is (among others) the moderncv class. These and some more document classes are discussed at What are the available "documentclass" types and their uses?.
Questions about specific classes are better tagged with the class-specific tag. For example, memoir questions should be tagged memoir and KOMA script classes should be tagged koma-script.
