Always, Always, ALWAYS click on the "logout" button in webmail, especially if you are on a public computer.
Our mail interface at our school is susceptible to the "referer bug" which basically means that if one is using webmail and clicks on an embedded url in an email, the server that is hosting that site can read the HTTP_REFERER variable and have access to your inbox. To check if you are vulnerable to this bug, copy the URL the next time you are using webmail, and email it to someone. Ask them to click on it within the next 5 minutes and see if they can access your inbox.
Basically, the way it works is this:
1. When you use webmail, if you notice, the URL contains the session id (sid) that is all you need to access your inbox. For example, mine is something like:
http://webmail.drexel.edu/en/mail.html?sid=gsgeny3gNgk&lang=en&cert=false
With this information, anyone can access your inbox until your session expires (60 minutes later) or you click logout.
2. When you click on an embedded link (one that is in your email), the server gets a variable called HTTP_REFERER that tells who is accessing their website. All they have to do is copy that URL into their browser, and they will be able to access your account.
Moral of the story: Never click on an embedded link in webmail. Always copy and paste it into the browser URL box.
If you are using a shared computer, don't forget to click "logout". Otherwise the next person that uses the internet browser can just access the last sites that someone visited and scroll down to any of the unexpired webmail sessions, and get into anyone's mailbox.
Here's a more technical explanation:
http://www.cotse.com/mailing-lists/bugtraq/2000/Jan/0132.html