An audit trail was originally the paper trail required to do an account audit, but is now used generically for any log of the process used to arrive at the current state, which can be inspected later for detailed information on how you got there.

Thus a manager might say "Joe used to produce those figures. We have his figures for this year, but he's left the company and there's no audit trail of his process, so I don't know how we'll do it next year".

In computer security, an audit trail is often used to track the actions of users and applications, and is used to detect security breaches after the fact. Thus a computer security expert might say "the honeypot server has a covert audit trail for all logins".

Or an audit trail can simply be a history of actions taken, e.g. "Everything2 has rigorous audit trails for submissions and deletions, but none for edits. Wikipedia keeps full audit trails for all edits."