怎样理解oracle中的shadow process?

哪位能告诉我怎样理解oracle中的shadow process?它和server process有什么联系吗？
不胜感激！！！！！！　　　　　　

I think oracle processes include two type:user processes and server processes.(maybe have another one named multiplexing processes).They call server processes as shadow processes,I don't know the difference of them!

The server processes communicate with the user and interact with Oracle to carry out the user's requests.If the user process requests a piece of data not already in the SGA, the shadow process is responsible for reading the data blocks from the datafiles into the SGA.