A site collection is administered by site collection administrators. You pick a primary and secondary site collection administrator when a site collection is first created. You can add more administrators afterwards if you like and this video shows you how to can do that. Keep in mind that site collection administrators can administer any site within the site collection even they have not been explicitly given permissions to a site.
*Applies to SharePoint 2010
Watch the video on Setting up Site Collection Admins that explains this functionality.
This video is available for free until 9/2/2010 then it will be returned to subscribers only area.