Cookiejacking is a technique used to help gain unauthorized access to accounts on popular web services.
Web browsers store certain information for particular web services (e-mail, social networking and so forth) when users login and use them. While they don't usually store actual login information (username and password), they are used by the browser (in conjunction with the web service) so enable a user to stay logged in. This is how you can tick a "Remember Me" box and avoid logging in to a web service for long periods of time, even if you restart your browser or computer.
Stealing "cookie credentials" and using them to gain unauthorized access to an account is not a new attack method, it has been around as long as cookies stored this type of information. However, it has not gone away, and in conjunction with spoofing techniques, cookiejacking can really be a problem.
Researchers have demonstrated ways in which some popular services, like Facebook or Twitter, can be effected when an attacker is on the same network as a victim. Certain security flaws in web browsers do not help either. One such flaw, reported in May 2011, affected Internet Explorer. If a user could be convinced to click a link within Facebook, and then drag and drop a virtual object with a pointing device, their cookie credentials could be stolen (read more here.)
While Cookiejacking may not be as effective as keyloggers or other methods for gaining unauthorized access to user accounts, it is still found prevalent in the wild.