Site Collection Administrators
Site collection administrators have full rights to all sites within a site collection. They can add or delete sites or change the settings for any site within a site collection. They can view, add, delete, or change all content within those sites as well. They can add and remove users from sites and send invitations to those sites.
Site Collection Owners
Site collection owners and secondary owners are also site collection administrators. However, owners and secondary owners are the only users who receive e-mail notifications for events, such as the pending automatic deletion of inactive sites. By default, site collection owners also receive requests for access from users who have been denied access. A user who is a member of the Administrator site group can use the Site Administration pages to change the e-mail address that receives requests for access.
When you create a site collection, Windows SharePoint Services automatically lists you as the site collection owner.
For more information, see http://technet.microsoft.com/library/cc288243(office.12).