go to this database concepts guide which explains to you about all different components of a oracle database.
http://download.oracle.com/docs/cd/B19306_01/server.102/b14220/toc.htm
If you want to find explanation about all oracle dictionary tables and initialization parameters then read this document
http://download.oracle.com/docs/cd/B19306_01/server.102/b14237/toc.htm