Let's not go in the very technical details but he used some sniffing tools called Ferret (to copy the GMail cookies to his computers) and Hamster (to use the cookies in his browser).
What can you do to prevent someone else from reading your GMail or Yahoo Mail?
Rob's method works when you are using the HTTP mode to access your email (http://www.gmail.com/). Therefore the trick is to always use Secure Login.
See: Recovering a Hacked GMail Account
Here's what you can do to safeguard your email in public wi-fi hotspots - use https:// instead of http:// - the entire session will be encrypted and the cloning cookies method will fail.
For GMail: https://mail.google.com/mail/
For basic HTML version of GMail - https://mail.google.com/mail/?ui=html
Alternatively, you can install the CustomizeGoogle extension of Firefox that will always force the SSL mode in GMail incase you forget to manually type the https:// GMail URLs.
Related: Recover Yahoo! or GMail Passwords