Every object in SQL Server has to be owned by someone. Ownership is used to control permissions. For example, any object that you own you have direct access to without it having to be granted to you. An other example is a procedure that has access to any object owned by the same owner. This allows someone with only access to the procedure to execute it even if it is accessing objects that are owned by someone else.
A schema is a grouping mechanism that most other RDBMSs dont have. (If an other RDBMS refers to a "schema" it means something more like what we call a "database".) Schemata allow to group objects of different types together into logical units within a single database. By default, the ownership of the schema is passed through directly to all objects in that schema. If you change the owner of the schema, the owner of all objects within changes too. (this can be changed on an object by object basis however.)
On the other end, you can grant access to a schema which in turn grants access to all objects within. This makes managing permissions a lot simpler.
Be aware, that you should always fully qualify every object with schemaname.objectname. The reason for this is two fold:
- Performance: SQL Server tries to figure out the schema of an object that was referenced without schema specification based on different settings and rules. This burns precious cpu cycles unnecessarily.
- While rare, it can lead to security and other access problems because the wrong object might be selected by SQL Server if multiple objects with the same name exist in different schemata.