Post-Workout supplements are used immediately after training. The post-workout period is the most vital time for ensuring your nutrition is perfect. It is at this time that your muscles are most sensitive to the nutrients you deliver them. What you feed them then is critical, and it is vital you give them the right balance of protein, carbs and other nutrients to facilitate the rapid replenishment of glycogen and transition your body into a positive nitrogen balance to aid muscle and joint repair.
Post-workout supplements are designed to deliver this in one package. The post-workout category is limited to products which are specifically formulated for post workout use but most products in the complete formulas, protein or creatine categories can be taken at this time as well to maximise their effectiveness.
For more information, take a look at this article: http://www.predatornutrition.com/Post-Workout-Nutrition