Pinkberry specializes in Asian-style tart frozen yogurt, which has the taste of real yogurt, slightly sweetened.  The effect is similar to sweet cream ice cream - a pure, creamy background for whatever toppings you want to add.  The best toppings they offer, IMHO, are the mochi and the fresh fruit - mangoes, strawberries, blueberries, kiwi, among others.     
It has spawned a whole series of competitors and has been the hot trend of 2006-07 in Southern California.  It will be interesting to see how much staying power they have.