When I was a kid, my father worked for Saudi Arabian Airlines. As a result, my friends and I spent a lot of time hanging out in the airport. As was common with boys back then, we had a fascination with all things airplane related, so we watched the activities of the airport. One of the things I observed were the arriving airplanes. Immediately upon arrival at their parking spot (there weren't jetways in Saudi back then), a swarm of crewmen began to work away on the airplane, even while the passengers where leaving the plane. On the larger airplanes that just had a long flight, the plane would move to a hanger where even more maintenance men did extensive maintenance. These planes would spend at least several hours in the hangar, before being returned to a parking space. The guys who worked there explained to us that after a flight, the planes needed to be worked on to clean up the systems and repair any minor damages before they can fly again. They said the bigger planes needed much more work because they had to fly for a much longer time therefore the plane must really be inspected carefully so they had to go to the hangars. But one particular model I noticed always seemed to spend a much shorter time in the hangar - sometimes only thirty minutes, the Lockheed L1011. The guys explained to us, Lockheed designed the L1011 centered around very quick turnaround maintenance.
Our bodies work the same way. Any physical activity - it really makes no difference what the activity is; moving heavy boxes, running, cycling, weightlifting, hot sex - microscopic damage occurs and metabolites and other wastes build up. Our bodies need to recover from such activities, and the longer and/or more intense the activity was, the longer the recovery period needs to be. Soreness that develops after weightlifting or other heavy use of muscles - often referred to as delayed onset muscle soreness, or DOMS - is the result of microscopic tearing of the muscle tissue. Lactic acid also builds up in the muscle tissue limiting the muscles' performance. You should note that these issues are not problems; it is what triggers our bodies to grow strong, tougher, and faster. But like the airplanes after their flights, our bodies need to recover to experience the growth. Rest/recovery is described as the third component of the three components of fitness, the other two being training and eating. Though some trainers and fitness experts consider rest the most important, in my view all three are equally needed. After all, eating right and resting well do little if the training is lacking and training hard and resting well will not accomplish much if one eats only junk, or way too little. So it follows that superior training with excellent eating habits also will minimize results if one does not allow enough time for rest and recovery. This is a mistake many people make, new and experienced; I have in my zeal forgotten to rest properly.
The beauty of rest is its simplicity; for the most part you do nothing. That is, you minimize physical activity as much as practical. This will help muscles repair. After any workout or other strenuous activity, drink plenty of water - you should already be keeping yourself well hydrated - as the body loses more water during exertion. Eat well also, the body needs the nutrients and protein to repair and grow. Finally, sleep, sleep, did I mention sleep? Our bodies do the most protein synthesis during sleep; that equates to better and faster recovery and growth. At the very minimum, get six hours, eight hours is better. I've read competitive bodybuilders go for as much as 9 or 10 hours. If you are or plan to become very active (working out more than 5 days a week), like the L1011 airliner, you will want to plan your lifestyle with recovery in mind. This means taking certain supplements immediately after a workout and/or prior to sleep. A common practice for active recovery is to take glutamines, creatine, whey, and a high glycemic carb immediately post workout. This combo works to help the body recover quicker and replenish the muscles' energy levels. I like to mix this combo with frozen blueberries and one and a half cups of milk for a shake. Prior to bed, some people will take glutamine to help with muscle growth and repair, and arginine to clean out the metabolites. Such practices helped me sustain a regimen of doing heavy cardio twice a day that helped me lose six pounds a week. Other things you can do for recovery; a nice soak in a hot tub (or bath if you don't have access to one) relaxes the muscles. Lately I have been getting a professional massage every couple of weeks, and it does help tremendously.
It is important to rest the body after you have been active. Growth and physical fitness improvements do not occur during training, but during the rest afterwards, especially during sleep. How well you rest will determine how well spent your efforts were in the gym. Give them hell at the gym - go all out balls to the wall. But when you are finished, take it easy!