regular np you can use to trade things with other namepro members but domain credit np you can only use to buy domain names from namepros, you cant even give it to other namepros members, hope that helps.
Like Paul has already said, NP$ may be used to sell/buy or trade with other NP'rs...while domain credits may not be transferred from one account to another and are only good for NP domain regs or on the Credit for Clicks program.