owners group is for permissions, think of it as being a security principle. Site Collection administrators are people who are the people who really own the site, wheras owners group is one of the groups that you can add users to. Search and rea about how you would add people to Owners gropu and then search and read about how you would make people site collectino administrators. These two different processes will make it clear to you.
This article about Site Collection Administrators has some explanation:
For you, as a site collection administrator, this means the following:
You might be expected to be the main point of contact between your part of the organization and your IT department.
You’ll need to work closely with people in certain roles in IT, such as your SharePoint farm administrator.
You’ll be performing some tasks that might previously have been referred to your organization’s IT team, such as deciding who has access to important intellectual property stored on your organizations web sites (that is, setting site-collection level permissions), and deciding which features to make available to the people who will be using the sites in your site collection.
You’ll probably be providing some technical support for the people who use your site collection.
You can read aboutb Owners grouo here:
The difference will be clear when you read these two articles.