A site is an organization account.
Within a site you can create one or more circles. It is the circles that contain Product Backlogs, teams, milestones etc.
Most users belong to one site and have access to one or more circles within the site. It is possible though, to have access to more than one site.
At site level you create the circles and define the relationships between circles (in the Circle Map).
In the Dashboard view at site level, you can see items and activity log across circles within the site.