The preamble is the first section of an input file, before the text of the document itself, in which you tell LaTeX the type of document, and other information LaTeX will need to format the document correctly. The preamble is used in "full blown" LaTeX, but not in the <math> . . . </math> tags in a wiki.
The LaTeX language consists of a "preamble" followed by "document text".
\documentclass{class}
… your preamble goes here …
\begin{document}
… your text goes here …
\end{document}
The document classes include article, book, letter, report, and slides.