Basics: Meal Replacement vs. Post-workout Shake

One workout standard is the "protein shake" - a mix of protein powder, liquid (usually, milk or water, sometimes juice), and other ingredients.

You'll occasionally see them as a powder or pre-mixed, and labeled either a "meal replacement" or "post-workout" or "energy" shakes.

What's the difference?

In a nutshell:

Meal Replacement: Mix of proteins (whey, casein), moderate to low carbs, some fat. Preferably includes fiber. Used to replace a regular meal with a liquid meal.

Post-workout shake: Whey protein, possibly plus carbohydrates (if you aren't going to add your own). Little fat or fiber.

Energy shake: Some whey protein, lots of carbohydrates. Little fat or fiber. The carbs are largely sugar.

How do you use them? It's pretty simple - meal replacements are aimed at replacing a solid meal with a liquid one. Post-workout shakes are intended to maximize the benefits of a workout with a quick-digesting meal of protein and carbohydrates. Energy shakes/drinks are for pre-workout boosts in energy - at least the better ones are. The not so good ones are largely just sugared drinks.

That's just a very basic overview.