What is the difference between a user and a schema?
Technically, a schema is a collection of database objects owned by a specific user. Those objects include tables, indexes, views, stored procedures, etc. In Oracle, a schema requires a user to be created. But you can create a user that has no schema (i.e, no objects). So in Oracle, the user is the account and the schema is the objects. It is possible in other database platforms to have a schema without a user.
More information on schemas in Oracle:
Relationship between schema and datafile in Oracle
How to create a database schema in Oracle?
Why Schema.org wants to standardize tags