The P90X fitness program gives you such an extensive array of different exercises that by the time you have practiced them and eventually master them, you will have done the workouts many times and challenged your body in many ways which leads to new muscle growth. By confusing the muscles your body has to increase its workload to adapt so you see changes faster.
The P90X avoids the dreaded fitness plateau by switching things up constantly. Every day for 12 days you do a different routine. You never adapt so it never gets easy.
More exercises on these 13 DVD's than you can imagine:
Plyometrics - Heisman Hops, Prayer Squats, Frog Jumps
Strength - Pull Ups, Push Ups
Yoga - Triangle, Warriors
Kickboxing- Jab, Cros, Hook, Side Kick