Here are the ones that I know about:
DVDR
CDR
Hard-Drive

I had some questions about each one
[DVD] - I don't plan on getting a burner anytime soon

[CDR]
Do games actually fit on CD's?

[Hard-Drive]
How much space is on the stock hard-drive?
What do I need to install a new hard-drive?
Can any PC hard-drive be used?

I would really appreciate any help on these newbie questions.