An audit log contains all the information necessary to follow a user's interaction with a system. It will usually contain more information and detail than what is necessary to have in day-to-day operations.
You can think of it in terms of a phone system. You can record every call that passes through the phone system to be able to go back in time and prove exactly what a person said, but it wouldn't be feasible to use a log containing all of that information for taking care of phone system operations.
An audit log usually contains more sensitive information than other system logs so access to it should be more restricted.