Oracle process overview. There are 3 types of oracle processes:
USER processes
SERVER processes
BACKGROUND processes

User
connected to a user application (eg sql*plus)
controls the application
associated with an oracle server process

Server processes
there are 2 types of oracle server process, depending on configuration
Dedicated ( 1 server process per user process )
Shared ( user processes are handled by a shared pool of server processes)

Background processes
to accommodate many users and maximise performance, oracle runs several background processes
there may be many background processes, but not all are always present.