Here is Oracle 11g architecture in one image, taken from official Oracle site.
- Processes shown are started by default
- Processes in red are mandatory
- Processes ARC and RVWR are not started by default but are recommended for productions
- Processes marked with * are new in 11g
For those who want full information, here is magnificent flash that explain many thing with details ...
Hope will help someone as it is to for quick reference.
This information are taken from official Oracle site and may be downloaded from original Oracle location.