GMail (and some other email providers) supports the ‘+’ directive in email addresses.
Thus, if your GMail address is:
Then, emails sent to the following address will reach you as well:
Based on the email address sent to, you can set automatic spam filtering or other rules.
This is handy for signing up on web sites, since you can choose a new email address for each, and then if you are being spammed deal with it (and know who it was that handed out your details).
It’s also handy for creating multiple test accounts when testing your web site as different users.
(Note: unfortunately a few web site validators don’t like the ‘+’ character, so this won’t always be an option)