If all else fails read the documentation. Try here for starters (v. good diagram). In Oracle, a schema is a database. Also see here - particularly this:
One characteristic of an RDBMS is the independence of physical data
storage from logical data structures. In Oracle Database, a database
schema is a collection of logical data structures, or schema objects
[i.e. related tables]. A database schema is owned by a database user
and has the same name as the user name.
A further distinction is between the physical database files "the database" and the "instance" - the running software that makes up the fully working system.
In one sense, an Oracle "database" is useless - it's just a bunch of bits on a disk - it's the software which turns it into a system (i.e. a database in the sense that information is retrievable/modifiable).
An Oracle database is a running instance plus data (+ control + redo &c.) files which itself may contain 1 or many schemas which are themselves "databases" - i.e. as the layman would understand them - i.e. a HR, CRM or ERP (&...) schemas/systems.