Mass gainers contain a balance of the three macronutrients (protein, carbs, fat) and tend to be rather high in calories to support a caloric surplus. Protein powders on the other hand contain mostly protein with a small amount of carbs and fats. They are designed to be lower in calories and added to other foods or beverages. Mass gainers are best used during muscle-gaining phases, whereas protein powders can be used during muscle-gaining, fat loss, and maintenance periods.