Letters
Synopsis:

\documentclass{letter}
\address{sender address}
\signature{sender name}
\begin{document}
\begin{letter}{recipient address}
\opening{salutation}
  letter body
\closing{closing text}
\end{letter}
  ...  more letters ...
\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}{Mr. Joe Smith \\ 
      2345 Princess St. \\ 
      Edinburgh, EH1 1AA}
   ...
 \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; it also can contain multiple lines.
Each letter environment 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 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 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 signature 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}{2015-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}

Specifies 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: without the \address declaration the letter is formatted with some blank space on top, for copying onto pre-printed letterhead paper. (See Overview, for details on your local implementation.) With the \address declaration, it is formatted as a personal letter.
Here is an example.

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

\cc
Synopsis:

\cc{first name \\ 
    .. }

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. Separate multiple lines with a double backslash \\.

\cc{President \\
    Vice President}

\closing
Synopsis:

\closing{text}

Usually at the end of a letter, above the handwritten signature, there is a \closing (although this command is optional). 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 each page. It only appears if the page style is firstpage.

\makelabels
Synopsis:

\makelabels

Create a sheet of address labels from the recipient addresses, one for each letter. This sheet will be output before the letters, with the idea that you can copy it to a sheet of peel-off labels. This command goes in the preamble.
Customize the labels by redefining the commands \startlabels, \mlabel, and \returnaddress 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{sender address}{recipient address} produces the two labels (or one, if you choose to ignore the sender address). The sender address is the value returned by the macro \returnaddress while recipient address is the value passed in the argument to the letter environment. By default \mlabel ignores the first argument, the sender address.

\name
Synopsis:

\name{name}

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

\opening
Synopsis:

\opening{text}

This command is required. It starts a letter, following the \begin{letter}{..}. The mandatory argument text is the text that starts your letter. 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 and after a vertical space for the traditional hand-written 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 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, for instance with \signature{\vspace{-6\medskipamount}\includegraphics{sig.png}\\ My name} (this requires writing \usepackage{graphicx} in the preamble).

\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.