An Oracle Question

Hi all,
I'm new to oracle, I've an oracle question that I would like to ask all of you. I was reading some Oracle materials and Im quite confused about the differences between redo log buffer and rollback segments. By the look of its description, both of them seem to have similar tasks... can anyone plse tell me the differences between the two?...are they related at all?? ...thanks in advance...
Very confused,
newbie

redo log buffer :stores the changes made to the data blocks in memory and when certain trigerring event occurs its content are flushed to the online redo log files on the disk.

rollback segments:contain before image of the data

lemme explain with an example

when you say

update emp
set dept=cia
where name='kelly';

the value of the rows containing kelly as the name before making this change are copied to the rollback segments.

when the changes are finally made these are written to the redo buffer.

Well , here is some more explaination ...as per the oracle documentation says , the before and after images are witten to the redo logs .once the redo logs have been applied in recovery process , and this means recovering the rollback segment entries in the buffer cache that may have been lost in a database crash -the rollback segments are applied-but the UNDO informati0on comes from the redo logs firtst .So the redo logs contains both undo and redo information .
Hope that doesn's confuse .