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")...