There are fun ship dollars and funship cruise certificates. Both have expiration dates. Funship dollars can be used toward anything, certificates can only be used towards the purchase of a cruise, and cannot be used if you go through a TA.
I've found funship dollars to be available only through Carnival, wherea the certificates are commonly available through programs run by third parties, such as giftcertificates.com and given away as rewards through different companies.
If you read the backs of each certificate they state the Terms and Conditions of use for fun ship dollars and certificates. They are quite different.