IIS 7 IISAppPool lightly explained
If you wonder about what IISAppPool is, please follow this link: http://learn.iis.net/page.aspx/624/application-pool-identities/ or this same one with an easier to read URL.
It is an almost-user that IIS uses for running each web in its own space. One can do the same with ordinary users but it seems to be to much of a hassle for us regular developers.
The article doesn’t explain everything; but exactly what I needed. Then together with http://stackoverflow.com/questions/4877741/access-to-the-path-is-denied and especially the comment by Hans Passant it made me solve an access problem.