Oracle手册上对于Oracle process的定义：
A unit of execution that runs the Oracle database code. The process execution architecture depends on the operating system. Oracle processes include server processes and background processes.
Oracle process即是运行Oracle数据库的代码，它分成server process和background process两种。
Server process的定义：
An Oracle process that communicates with a client process and Oracle Database to fulfill user requests. The server processes are associated with a database instance, but are not part of the instance.
Server process可以看做是连接数据库client进程和数据库之间的桥梁，用来处理client的请求。虽然同Instance相关联，但不属于具体的Instance。
Background process的定义：
A process that consolidates functions that would otherwise be handled by multiple Oracle programs running for each client process. The background processes asynchronously perform I/O and monitor other Oracle processes.
Background process用来做实际的I/O操作和监控其它Oracle进程。
参考资料：
Glossary。