Oracle is a Relational Database Management System or RDBMS this means it contains related structured data in database objects known as tables.
The original incarnation of the Oracle database was built by Lawrence Ellison and a team of developers, Lawrence or Larry is now CEO of Oracle Corp.
It is built around a relational framework from which database objects can be accessed by other applications or users. To access these items you use SQL.
Oracle is used by global enterprises and shares the vast majority of the database market with Microsoft and their SQL Server offering – both Oracle & SQL Server have very similar structures.
To understand it better, consider the database to be a filing cabinet – each drawer is a schema within that database, the files within the drawers are the tables within each schema and the pieces of paper within the files is the data.
The following subjects are more for information than anything else – developers do not usually get involved in creating or managing the main components of the Oracle database i.e. tablespaces, schema’s, users & privileges etc.