You can use LaTeX to typeset letters, both personal and business. The letter document style is designed to make a number of letters at once, although you can make just one if you so desire.
Your .TEX source file has the same minimum commands as the other document styles, i.e., you must have the following commands as a minimum:
\documentstyle{letter}
\begin{document}
... letters ...
\end{document}
Each letter is a letter environment, whose argument is the name and address of the recipient. For example, you might have
\begin{letter}{Mr. John Doe \\ 2345 Jones St.
\\ Oakland, CA 91123}
...
\end{letter}
The letter itself begins with the \opening command. The text of the letter follows. It is typed as ordinary LaTeX input. Commands that make no sense in a letter, like \chapter, don't work. The letter closes with a \closing command.
After the closing, you can have additional material. The \cc command produces the usual "cc: ...". There's also a similar \encl command for a list of enclosures.