Audit Log Reporting seems to be one of those topics within SharePoint that is still a little misunderstood. I have blogged on it before here , and suggest that if you are not familiar with Audit Logs that you view that post quickly before reading on....
I had a forced introduction to Auditing Reports, a subject which isn't very well convered pretty much anywhere. What is it? A history of all actions taken on a site collection since the creation of the site collection. Where is it stored? Dbo.auditData...