Gmail is Google's free email service. It lets you create additional email addresses, which can be accessed via your web browser.
Google has a business partnership with AOL (and is part-owner of it). It seems (from your post) that AOL is now providing a direct link from their home page to access your GMail account. (If you've not yet got a GMail account, the link will probably take you to a sign-up page).
Some people don't like the way that GMail scans all of your mail in order to append advertising material:
http://www.privacyrights.org/ar/GmailLetter.ht m
Chris