article: this is the most important style, since it was the primary design used when LaTeX was created. You should use article as the style whenever you want a simple document without special styles. It is possible to extend the article style with several options as described later.

book: this style should be used to format documents with the characteristics of books: double-sided, chapter subsections, table of contents and index. The format provides all these commands in a simple way.

report: the report format has many of the features of the book style, but with simpler formatting options. It is generally used for technical reports such as dissertations and monographs.

The styles presented above can also be combined with optional parameters. These parameters can be used to activate particular features of a style, which would otherwise require manual intervention. Here are some examples of optional parameters for \\documentclass:

twocolumn: prints the document in two columns, instead of a single column, which is the default. This is useful in articles, for example, where the object is to fill more content on a single page.

fleqn: locates all equation numbers on the left size of the page, as opposed to the default of numbers in the right margin.

a4paper: used to defined the paper size to be a4, instead of legal (default in the English-speaking countries).

11pt: changes the default font size to 11 points, instead of the default 10 points. There is also a similar option 12pt that changes the default font to 12 points.