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
report classes. KOMA script offers replacements for those standard classes:
scrrprt. To find out about differences between these classes, see book vs. report vs. article document class. The
memoir class is another popular choice.
Other, less common classes used for special purposes include the
letter class and its KOMA replacement
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?.