Good question. To be described as "leather" then the seat must be made of animal hide. However, it doesn't mean the whole seat is leather. When you see phrases such as "leather seat facings" you'll often find that the middle of the seat is leather but the outer parts are synthetic. Look for phrases such as "leather effect" and "PU leather" in sales literature as these are subtle ways manufacturers use to disguise the fact they're using cheaper materials. If you want to avoid leather for personal reasons then you should always look for a car with cloth or velour trim - some manufacturers (such as Jaguar) even offer a vegan alternative. Incidentally, car interiors account for over 30% of the world's leather production, second only to the shoe industry.