You may have seen a link in your eventlog that, when you click on it, takes you to a microsoft web site where theoretically there is supposed to be useful information telling you what to do about that specific event, but often there is not.
If this has happened to you enough times that you stopped clicking on the links, hear my plea: Keep clicking! The reason is that we look at the hit count of these web hits to determine the priority of the events we should document in our event lookup (here for E2K, here for E2K3 - both listed on this page).
Also note that for Exchange 2003, there was a big effort to document many more events than we used to, so hopefully the E2K3 event lookup has helped you more than the E2K one did. If you have a specific event that you were looking up that did not have content in the eventid lookup, add a comment here and I'll put together the feedback and send it on to the people who own that site.
Also, I should add that the above isn't the only way we document events. Many events are documented in the KB, and those may come from bugs in our database, customer reports, etc. We could do a better job of synchronizing the content between those two back-end systems though.