This method is a bit less known but in recent months it has become widely popular due to the introduction of the software 'FIRESHEEP'. When you log in to your account at a website, your web browser sends your username and password to the website server. The password is first encrypted and then send over the network. Then the server checks the username and password against the database and if they both match then sends to the user's browser a "cookie" (a text which the browser uses for further requests to the web server) but unlike the password the cookies are not encrypted and are sent as it is over the network, this cookies can be easily captured as they travel through the network or a busy WI-FI. The newly released Firefox extension "FIRESHEEP" makes it more easy. Actually so easy that even a total nube can capture this cookies.