If your site has multiple admins, you may want to turn on the admin action log, just so everybody can keep track of who did what, when.
There are three logging levels: off, basic, and extended, represented in the variable use_logging as the numbers 0, 1, and 2, respectively. With logging off, as you'd expect, nothing is logged. With basic logging turned on, the user, date, and action are logged. With extended logging turned on, everything logged in basic logging plus the contents of the item an admin dealt with (such as the text of a deleted comment) are also recorded.
To determine which admin actions are logged, you must set them in the Hooks Admin Tool (A.18). The hooks directly recognised and handled specially are: comment_delete, story_new, story_update, and story_delete. If you wish to log any other activities, the logging tool will log up to two arbitrary pieces of information it is given. For all action logging, a hook must bind to the function log_activity and be enabled. The two pieces of information logged by the log_activity function for actions other than those listed above are the first two parameters sent by the hook definition.
This features is primarily meant to log administrator actions, not everyday user actions. If you want to log every move every visitor makes, use the variable paranoid_logging and read your server logs. Generally, the only actions you will log are those restricted to administrators, such as deleting comments.