Answer: It is not what most people are led to believe by the popular media and most supplement companies. For some reason (and it isn't scientific evidence or human physiology) most drinks promoted for recovery or post-workout are exactly opposite of what is appropriate. Commonly marketed drinks and bars tend to be very high in protein and low in carbohydrates. While it is true that you need some protein after a workout, carbohydrate should be present in an amount that is 2-4 times greater than protein. This is essential for the stimulation of insulin, the hormone that ignites the recovery and building process, putting the protein to good use. Recent research has shown that immediate pre- and post-workout nutrient intake (best accomplished with a liquid meal, although bars will work well) increases muscle recovery and muscle building to a greater extent than any other timing or eating strategy for nutrient intake. You want to consume a drink of predominately carbohydrates made up of a mixture of sugars, 10-20 grams of protein and very little fat.
For more information on creating the ultimate environment for building muscle see Xtreme Muscle Stack: Creating the Perfect Anabolic Storm.