从一个国外开源CMS系统找出来的邮件发送类，中间的有点儿不明白！
大家帮忙分析一下：

[php]
<?php
class PHPMailer
{
/////////////////////////////////////////////////
// PUBLIC VARIABLES
/////////////////////////////////////////////////

/**
* Email priority (1 = High, 3 = Normal, 5 = low).
* @var int
*/
var $Priority = 3;

/**
* Sets the CharSet of the message.
* @var string
*/
//var $CharSet = "iso-8859-1";
var $CharSet = _CHARSET;
/**
* Sets the Content-type of the message.
* @var string
*/
var $ContentType = 'text/plain';

/**
* Sets the Encoding of the message. Options for this are "8bit",
* "7bit", "binary", "base64", and "quoted-printable".
* @var string
*/
var $Encoding = '8bit';

/**
* Holds the most recent mailer error message.
* @var string
*/
var $ErrorInfo = '';

/**
* Sets the From email address for the message.
* @var string
*/
var $From = 'root@localhost';

/**
* Sets the From name of the message.
* @var string
*/
var $FromName = 'Root User';

/**
* Sets the Sender email (Return-Path) of the message. If not empty,
* will be sent via -f to sendmail or as 'MAIL FROM' in smtp mode.
* @var string
*/
var $Sender = '';

/**
* Sets the Subject of the message.
* @var string
*/
var $Subject = '';

/**
* Sets the Body of the message. This can be either an HTML or text body.
* If HTML then run IsHTML(true).
* @var string
*/
var $Body = '';

/**
* Sets the text-only body of the message. This automatically sets the
* email to multipart/alternative. This body can be read by mail
* clients that do not have HTML email capability such as mutt. Clients
* that can read HTML will view the normal Body.
* @var string
*/
var $AltBody = '';

/**
* Sets word wrapping on the body of the message to a given number of
* characters.
* @var int
*/
var $WordWrap = 0;

/**
* Method to send mail: ("mail", "sendmail", or "smtp").
* @var string
*/
var $Mailer = 'mail';

/**
* Sets the path of the sendmail program.
* @var string
*/
var $Sendmail = '/usr/sbin/sendmail';

/**
* Holds PHPMailer version.
* @var string
*/
var $Version = '1.73';

/**
* Sets the email address that a reading confirmation will be sent.
* @var string
*/
var $ConfirmReadingTo = '';

/**
* Sets the hostname to use in Message-Id and Received headers
* and as default HELO string. If empty, the value returned
* by SERVER_NAME is used or 'localhost.localdomain'.
* @var string
*/
var $Hostname = '';


/////////////////////////////////////////////////
// SMTP VARIABLES
/////////////////////////////////////////////////

/**
* Sets the SMTP hosts. All hosts must be separated by a
* semicolon. You can also specify a different port
* for each host by using this format: [hostname:port]
* (e.g. "smtp1.example.com:25;smtp2.example.com").
* Hosts will be tried in order.
* @var string
*/
var $Host = 'localhost';

/**
* Sets the default SMTP server port.
* @var int
*/
var $Port = 25;

/**
* Sets the SMTP HELO of the message (Default is $Hostname).
* @var string
*/
var $Helo = '';

/**
* Sets SMTP authentication. Utilizes the Username and Password variables.
* @var bool
*/
var $SMTPAuth = false;

/**
* Sets SMTP username.
* @var string
*/
var $Username = '';

/**
* Sets SMTP password.
* @var string
*/
var $Password = '';

/**
* Sets the SMTP server timeout in seconds. This function will not
* work with the win32 version.
* @var int
*/
var $Timeout = 10;

/**
* Sets SMTP class debugging on or off.
* @var bool
*/
var $SMTPDebug = false;

/**
* Prevents the SMTP connection from being closed after each mail
* sending. If this is set to true then to close the connection
* requires an explicit call to SmtpClose().
* @var bool
*/
var $SMTPKeepAlive = false;

/**#@+
* @access private
*/
var $smtp = NULL;
var $to = array();
var $cc = array();
var $bcc = array();
var $ReplyTo = array();
var $attachment = array();
var $CustomHeader = array();
var $message_type = '';
var $boundary = array();
var $language = array();
var $error_count = 0;
var $LE = "\n";
/**#@-*/

/////////////////////////////////////////////////
// VARIABLE METHODS
/////////////////////////////////////////////////

/**
* Sets message type to HTML.
* @param bool $bool
* @return void
*/
function IsHTML($bool) {
if($bool)
$this->ContentType = 'text/html';
else
$this->ContentType = 'text/plain';
}

/**
* Sets Mailer to send message using SMTP.
* @return void
*/
function IsSMTP() {
$this->Mailer = 'smtp';
}

/**
* Sets Mailer to send message using PHP mail() function.
* @return void
*/
function IsMail() {
$this->Mailer = 'mail';
}

/**
* Sets Mailer to send message using the $Sendmail program.
* @return void
*/
function IsSendmail() {
$this->Mailer = 'sendmail';
}

/**
* Sets Mailer to send message using the qmail MTA.
* @return void
*/
function IsQmail() {
$this->Sendmail = '/var/qmail/bin/sendmail';
$this->Mailer = 'sendmail';
}


/////////////////////////////////////////////////
// RECIPIENT METHODS
/////////////////////////////////////////////////

/**
* Adds a "To" address.
* @param string $address
* @param string $name
* @return void
*/
function AddAddress($address, $name = '') {
$cur = count($this->to);
$this->to[$cur][0] = trim($address);
$this->to[$cur][1] = $name;
}

/**
* Adds a "Cc" address. Note: this function works
* with the SMTP mailer on win32, not with the "mail"
* mailer.
* @param string $address
* @param string $name
* @return void
*/
function AddCC($address, $name = '') {
$cur = count($this->cc);
$this->cc[$cur][0] = trim($address);
$this->cc[$cur][1] = $name;
}

/**
* Adds a "Bcc" address. Note: this function works
* with the SMTP mailer on win32, not with the "mail"
* mailer.
* @param string $address
* @param string $name
* @return void
*/
function AddBCC($address, $name = '') {
$cur = count($this->bcc);
$this->bcc[$cur][0] = trim($address);
$this->bcc[$cur][1] = $name;
}

/**
* Adds a "Reply-to" address.
* @param string $address
* @param string $name
* @return void
*/
function AddReplyTo($address, $name = '') {
$cur = count($this->ReplyTo);
$this->ReplyTo[$cur][0] = trim($address);
$this->ReplyTo[$cur][1] = $name;
}


[/php]