Who are the owners and co-owners of a record?

The owner of a record is the user in the Assigned To field. The co-owner of a record is the user in the Co-owners field. 
A record can have one or more co-owners depending on the value of the field - it can be either a user or a group. When the value is a group name, then all the users belonging to the group become co-owners.
To learn more about groups, click here.
Note: In the case of nested groups (where a group is a part of another group), all the members of the group get access to records.