I recently had a bear of a time trying to determine why a particular web application wasn't behaving like it should. As part of my log checking I was going over the HTTPErr logs in IIS in addition to going over the standard IIS logs and event logs.
While I have a good amount of experience pouring over and making sense out of event logs and IIS logs, I hadn't really looked at HTTPErr logs too much in depth until then and wanted to find out more...after a quick search I came up with the following Microsoft article which explains it all nicely.
I'm posting that link here so I can remember it in the future and hopefully it will help someone else out too! - http://support.microsoft.com/?id=820729