As with any complicated web application there are times when it is useful for the site to remember the state of the user interface. For example, if the user clicks 'Advanced' to open up a hidden section, when the user returns to this page the advanced section should remember its open state.
How it works
state.js is often used. For any checkboxes with the class 'chk' a jQuery function will be called which ultimately calls
set_state.php through AJAX to record state transparently in the background.
Cookies are still used for authentication purposes.