« fromage frais » is a very young cheese with a soft texture and taste. It looks very white. It had no time to dry. It ranges from "fromage blanc" which is nearly a yogurt to fromage "ail et fines herbes" or "chèvre frais" like on this photo :
« crème fraîche » is just the fat naturally collected on top of cow's milk. If you whip « crème fraîche » it becomes whipped cream or « crème chantilly », but if you whip it longer, then it becomes butter...
Some people use « fromage frais » instead of « crème fraîche » because it contains less fat. So if you're on a diet you can try.
Many French people rather use "crème fraîche" even if it contains a lot of fat, only because they're used to its taste.
If you want to cook a « gratin » or any kind of dish, you can use one instead of the other, just try, taste and
There's one thing you cannot do : have « crème fraîche » instead of « fromage frais » at the end of your meal. It would really be too fatty...