Channels are new in PEAR 1.4.0. Channels are a systemized way of differentiating between different download sources. One such download source is pear.php.net, another is pecl.php.net, and there are several third party channels slowly springing up around the net.
The <channel> tag should contain the full channel name, not any alias (don't use "pear", use "pear.php.net".
A good rule of thumb to use when determining what channel name to use is "where do I upload my releases to?" If the answer is pear.php.net, that is your package's channel. If the answer is pecl.php.net, then that is your channel.
If you aren't running a channel server and wish to serve your packages as well as allow other packages to remotely depend on your package, the new uri-based package distribution method is the best choice, see the <uri> tag documentation link below.