Difference between REDO and UNDO in oracle

Hi,

I have a few questions

1) What is exact difference between UNDO and REDO?
2) When did UNDO came into picture in which Oracle version?
3) Is it necessary to maintain UNDO and REDO together for my database installation?
4) What all are the physical files related to UNDO and what all are the physical files related to REDO?

Rgds,
Suhin