An Oracle user and scheme are the same?

Almost , but not exactly.

An Oracle user is the USERNAME, the SCHEMA is the set of objects associated with one OWNER

So when a USER become a SCHEMA? when he get the privileges to create objects;

Another thought, after login you are the user that you logon, but if you do an Alter session set current_schema=another_schema; 

Then you will be one user, but another schema.

Of course , to makes things easier to understand , you can think that they are the same.

One important thing, Inside Oracle dictionary tables just exists the USER