what happens in scheduling

Can some one tell me what exactly happens in scheduling...

I have gone through the OM user guide but couldnt understand anything..

There is a nice explanation

There is a nice explanation in meta link https://metalink2.oracle.com/metalink/plsql/f?p=130:14:6457879135351464710::::p14_database_id,p14_docid,p14_show_header,p14_show_help,p14_black_frame,p14_font:NOT,732914.1,1,1,1,helvetica