Letters
Synopsis:

\documentclass{letter}
\address{senders address}   % return address
\signature{sender name}
\begin{document}
\begin{letter}{recipient address}
\opening{salutation}
  letter body
\closing{closing text}
\end{letter}
   ...
\end{document}

Produce one or more letters.
Each letter is in a separate letter environment, whose argument recipient address often contains multiple lines separated with a double backslash, (\\). For example, you might have:

 \begin{letter}{Ninon de l'Enclos \\
                l'h\^otel Sagonne}
   ...
 \end{letter}

The start of the letter environment resets the page number to 1, and the footnote number to 1 also.
The sender address and sender name are common to all of the letters, whether there is one or more, so these are best put in the preamble. As with the recipient address, often sender address contains multiple lines separated by a double backslash (\\). LaTeX will put the sender name under the closing, after a vertical space for the traditional hand-written signature.
Each letter environment body begins with a required \opening command such as \opening{Dear Madam or Sir:}. The letter body text is ordinary LaTeX so it can contain everything from enumerated lists to displayed math, except that commands such as \chapter that make no sense in a letter are turned off. Each letter environment body typically ends with a \closing command such as \closing{Yours,}.
Additional material may come after the \closing. You can say who is receiving a copy of the letter with a command like \cc{the Boss \\ the Boss's Boss}. There’s a similar \encl command for a list of enclosures. And, you can add a postscript with \ps.
LaTeX’s default is to indent the sender name and the closing above it by a length of \longindentation. By default this is 0.5\textwidth. To make them flush left, put \setlength{\longindentation}{0em} in your preamble.
To set a fixed date use something like \renewcommand{\today}{1958-Oct-12}. If put in your preamble then it will apply to all the letters.
This example shows only one letter environment. The three lines marked as optional are typically omitted.

\documentclass{letter}
\address{Sender's street \\ Sender's town}
\signature{Sender's name \\ Sender's title}
% optional: \location{Mailbox 13}
% optional: \telephone{(102) 555-0101}
\begin{document}
\begin{letter}{Recipient's name \\ Recipient's address}
\opening{Sir:}
% optional: \thispagestyle{firstpage}
I am not interested in entering a business arrangement with you.
\closing{Your most humble, etc.,}
\end{letter}
\end{document}

These commands are used with the letter class.

\address
Synopsis:

\address{senders address}

Specify the return address, as it appears on the letter and on the envelope. Separate multiple lines in senders address with a double backslash, \\.
Because it can apply to multiple letters this declaration is often put in the preamble. However, it can go anywhere, including inside an individual letter environment.
This command is optional: if you do not use it then the letter is formatted with some blank space on top, for copying onto pre-printed letterhead paper. If you do use the \address declaration then it is formatted as a personal letter.
Here is an example.

\address{Stephen Maturin \\
         The Grapes of the Savoy}

\cc
Synopsis:

\cc{name0 \\ 
     ... }

Produce a list of names to which copies of the letter were sent. This command is optional. If it appears then typically it comes after \closing. Put the names on different lines by separating them with a double backslash, \\, as in:

\cc{President \\
    Vice President}

\closing
Synopsis:

\closing{text}

Produce the letter’s closing. This is optional, but usual. It appears at the end of a letter, above a handwritten signature. For example:

\closing{Regards,}

\encl
Synopsis:

\encl{first enclosed object \\ 
       ... }

Produce a list of things included with the letter. This command is optional; when it is used, it typically is put after \closing. Separate multiple lines with a double backslash, \\.

\encl{License \\
      Passport}

\location
Synopsis:

\location{text}

The text appears centered at the bottom of the page. It only appears if the page style is firstpage.

\makelabels
Synopsis:

\makelabels   % in preamble

Optional, for a document that contains letter environments. If you just put \makelabels in the preamble then at the end of the document you will get a sheet with labels for all the recipients, one for each letter environment, that you can copy to a sheet of peel-off address labels.
Customize the labels by redefining the commands \startlabels, \mlabel, and \returnaddress (and perhaps \name) in the preamble. The command \startlabels sets the width, height, number of columns, etc., of the page onto which the labels are printed. The command \mlabel{return address}{recipient address} produces the two labels (or one, if you choose to ignore the return address) for each letter environment. The first argument, return address, is the value returned by the macro \returnaddress. The second argument, recipient address, is the value passed in the argument to the letter environment. By default \mlabel ignores the first argument, the return address, causing the default behavior described in the prior paragraph.
This illustrates customization. Its output includes a page with two columns having two labels each.

\documentclass{letter}
\renewcommand*{\returnaddress}{Fred McGuilicuddy \\
                               Oshkosh, Mineola 12305}
\newcommand*\originalMlabel{}
\let\originalMlabel\mlabel
\def\mlabel#1#2{\originalMlabel{}{#1}\originalMlabel{}{#2}}
\makelabels
  ...
\begin{document}
\begin{letter}{A Einstein \\
               112 Mercer Street \\
               Princeton, New Jersey, USA 08540}
  ...
\end{letter}
\begin{letter}{K G\"odel \\
               145 Linden Lane \\
               Princeton, New Jersey, USA 08540}
  ...
\end{letter}
\end{document}

The first column contains the return address twice. The second column contains the address for each recipient.
The package envlab makes formatting the labels easier, with standard sizes already provided. The preamble lines \usepackage[personalenvelope]{envlab} and \makelabels are all that you need to print envelopes.

\name
Synopsis:

\name{name}

Optional. Sender’s name, used for printing on the envelope together with the return address.

\opening
Synopsis:

\opening{salutation}

Required. Follows the \begin{letter}{...}. The argument salutation is mandatory. For instance:

\opening{Dear John:}

\ps
Synopsis:

\ps{text}

Add a postscript. This command is optional and usually is used after \closing.

\ps{P.S. After you have read this letter, burn it. Or eat it.}

\signature
Synopsis:

\signature{first line \\
            ... }

The sender’s name. This command is optional, although its inclusion is usual.
The argument text appears at the end of the letter, after the closing. LaTeX leaves some vertical space for a handwritten signature. Separate multiple lines with a double backslash, \\. For example:

\signature{J Fred Muggs \\
           White House}

LaTeX’s default for the vertical space from the \closing text down to the \signature text is 6\medskipamount, which is six times \medskipamount (where \medskipamount is equal to a \parskip, which in turn is defined by default here to 0.7em).
This command is usually in the preamble, to apply to all the letters in the document. To have it apply to one letter only, put it inside a letter environment and before the \closing.
You can include a graphic in the signature as here.

\signature{\vspace{-6\medskipamount}\includegraphics{sig.png}\\
             My name}

For this you must put \usepackage{graphicx} in the preamble (see Graphics).

\telephone
Synopsis:

\telephone{number}

The sender’s telephone number. This is typically in the preamble, where it applies to all letters. This only appears if the firstpage pagestyle is selected. If so, it appears on the lower right of the page.