A SQL Server schema is simply container of objects, such as tables, stored procedures, etc. A Database Role is a group of principals, such as windows logins, sql server users, etc.
The idea is you can have a role of say “IT”, and have all IT users under that role. Then you have can a schema called “IT”, and have all tables that belong to IT under that. Out of the box SQL Server creates matching schemas for each default user and role in the database, but I think the intention is you customize this to match the needs of your organization.
This article has more information on the differences between owners and schemas. This question on Stack Overflow may also be useful.