By default Admins have access to the file system starting at /Data/Sites/[SiteID]
Users with general upload permission (from Site Settings > Security > Permissions) can access starting at
and users who have only roles that can upload and browse but only to a user specific location have
However it is also possible by a config setting to limit admins also to
<add key="ForceAdminsToUseMediaFolder" value="true" />
I do this for example on our demo site to keep people from messing things up there.
Hope that helps,