Oracle Database Discussions

Hi,
Sorry for asking very basic question. I know that DB cache and undo contain data blocks from physical datafiles, changes happen to them and reapplied to datafile again.
However, I want to know that in what form redo contain data, does it contain just text based sql statement to redo transaction activity or in other form. It would be helpful if any example based information can be given to understand complex redo activity.

Thanks in advance.