An Oracle database server consists of a database and at least one database instance in case of a single instance database. In case of Real Application Cluster, an Oracle database will have more than one instance accessing the database.
A Database is a set of files, located on disk, that store data. These files can exist independently of a database instance
An instance is a set of memory structures that manage database files. The instance consists of a shared memory area, called the system global area (SGA), and a set of background processes. An instance can exist independently of database files
The physical database structures that comprises a database are
Data files - every Oracle database has one or more physical data files, which contain all the database data. The data of logical database structures, such as tables and indexes, is physically stored in the data files.
Control files - every Oracle database has a control file. A control file contains metadata specifying the physical structure of the database, including the database name and the names and locations of the database files.
Online redo log files - every Oracle Database has an online redo log, which is a set of two or more online redo log files. An online redo log is made up of redo entries (also called redo log records), which record all changes made to data.
Many other files including parameter files, archived redo files, backup files and networking files are important to any oracle database operations
More information about Oracle Database Architecture can be found here.