Every Gmail account supports "plus addressing".
Every email address contains 3 parts. Let's call them the prefix, the @ sign and the suffix.
In email@example.com, bozo is the prefix and gmail.com is the suffix.
You can add "+anyword" to the prefix.
If my email address was firstname.lastname@example.org, I could use email@example.com on Amazon.com and firstname.lastname@example.org for Facebook.
Then I could set my email software to filter them into different folders based on the +anyword part of the address.
Many websites, however, won't allow you to sign up with a "+" in your email address, including Google Alerts.