I recently decided to crawl over the indexes on one of our most heavily used databases to see which were suboptimal. I generated the built-in Index Usage Statistics report from SSMS, and it's showing me a great deal of information that I'm unsure how to understand.

I found an article at Carpe Datum about the report, but it doesn't tell me much more than I could assume from the column titles.

In particular, the report differentiates between User activity and system activity, and I'm unsure what qualifies as each type of activity.

I assume that any query that uses a given index increases the '# of user X' columns. But what increases the system columns? building statistics?

Is there anything that depends on the user or role(s) of a user that's running the query?