What's the difference between a iphone 4 and a iphone 4 cdma? - 1

What's the difference between a iphone 4 and a iphone 4 cdma?
that has the iphone 4 that does not have the iphone 4 cdma?
is a good idea buy a iphone 4 cdma?
is a good idea pay 50 dollars and a blackberry 8530 (cdma )?
or better buy a ipod with something wrong (screem blok e or things like that )?

Cdma means it is either for verizon or sprint.