IMVU runs on credits, its own internal currency. Almost everything in IMVU is purchased with credits, except for some items purchased directly from IMVU, such as VIP memberships and AP (Access Passes).
 
There are two types of credits, Real credits (which have to be purchased with real money) and Promo credits, which are  given to you by IMVU. (Promo credits are also called predits). New users start out with a chunk of Promo Credits and can earn extra ones by playing games and doing various tasks.
 
From your point of view as a buyer, there is no real difference, when you buy some clothing, IMVU first uses your Promo credits and if there are not enough, it uses your Real credits.
 
So what are the differences?

• You cannot gift Promo credits to another user
• You cannot use Promo credits to buy a gift for someone
• You cannot sell Promo credits for real money

 
The real difference is for the developer that you buy products from. When you buy with real credits, the developer gets paid with real credits, when you buy with Promo credits - the developer does not get those credits!
 
Developers get one "Developer token" for a sale purchased with Promo credits - one only!- mo matter how much you paid. The developer token has very limited use, and its effectively worth only 10 credits,  so developers do not like them - so you may see a lot of grumbling about this.
 
Thre is not much you can do, since you cannot control whether a sales use Promo or Real (well you can by using a little trick, but we don't mention it here), so ignore any developer who says you have to pay with real credits.
 
For an intereresting discussion on why Promo Credits are needed see Keef's A basic guide to IMVU’s Currency and its Economy