When logging into a website you usually start by submitting your username and password. The server then checks to see if an account matching this information exists and if so, replies back to you with a "cookie" which is used by your browser for all subsequent requests.
It's extremely common for websites to protect your password by
encrypting the initial login, but surprisingly uncommon for websites to
encrypt everything else. This leaves the cookie (and the user)
vulnerable. HTTP session hijacking (sometimes called "sidejacking") is
when an attacker gets a hold of a user's cookie, allowing them to do
anything the user can do on a particular website. On an open wireless
network, cookies are basically shouted through the air, making these
attacks extremely easy.
This is a widely known problem that has been talked about to death,
yet very popular websites continue to fail at protecting their users.
The only effective fix for this problem is full end-to-end encryption,
known on the web as HTTPS or SSL. Facebook is constantly rolling out
new "privacy" features in an endless attempt to quell the screams of
unhappy users, but what's the point when someone can just take over an
account entirely? Twitter forced all third party developers to use
OAuth then immediately released (and promoted) a new version of their
insecure website. When it comes to user privacy, SSL is the elephant in
the room
