Branched Chain Amino Acids are a magical set of aminos called L-Leucine, L-Valine and L-Isoleucine. Ever wonder why you need protein to put on muscle? The correct answer is "Branched Chain Amino Acids". There are many aminos in animal and vegetable proteins, but it is just the BCAA's that signal your body to rebuild your muscles, initiate protein synthesis and promote recovery.
Below are some of the wonder-working powers of BCAA's for us males according to the research:
1) Protein Synthesis. BCAA's increase protein synthesis and decrease protein degradation in muscle tissue due to activation of certain key enzymes. 
2) Muscle Damage and Soreness. Several studies have shown that Branched Chain Amino Acids reduce muscle damage and soreness due to physical exercise. 
3) Immune Response Post-Exercise. In elite athletes, BCAA's were shown to preserve glutamine and immune function. 
4) Muscle and Strength. Several studies have shown that Branched Chain Amino Acids don't just work on paper: they actually increase muscle mass and/or strength. One interesting study found that consumption of BCAA's prior to a workout increased strength and mass more than consumption of whey, which is a favorite in the bodybuilding world.  By far the best alternative for whey is undenatured or biologically active whey, which I document in my link called Yes Whey if you want more information.
Well, one could go on and on about the near miraculous properties of these BCAA's when it comes to muscle. Now some guys might be asking, "Well, I don't want to look like Arnold. Who cares about putting on a bunch of muscle mass, anyway?"
To those naysayers, I have to explain that muscle mass is likely one of the keys to anti-aging. (It's also a key to your bedroom success and self-image.) Preserving your muscle is very critical, because "muscle burns fat". As we age, it's easy to simultaneously lose muscle and gain fat tissue. This reduction of our muscle to fat ration is very dangerous and makes it even easier to put on more fat. And extra body fat, as I document in my links on the Dangers of the Bulge and Nasty Visceral Fat, kills your health and your sex life.
However, Branched Chain Amino Acids do not just fight fat metabolism issues long term - studies show they fight them short term as well according to several animal studies. For example, one 2007 study on mice showed that in overweight mice on a high fat diet, doubling leucine resulted in a dramatic reduction in weight gain and reduction in fat tissues.  This has translated to human studies as well. A 2010 study of middle-aged people found that those with the lowest (by quartile) intake of BCAA's versus those with the highest had a 30% greater risk of being overweight.  In other words, BCAA's do just as billed: helping the middle aged and beyond "lean out" and win the battle the bulge.
How do Branched Chain Amino Acids work their magic? One way may be through impressive changes in blood sugar metabolism once one has Metabolic Syndrome or prediabetes. To demonstrate this, researchers gave mice a high fat diet, which induces insulin resistance, fatty liver disease and inflammation. They then doubled the mice's leucine intake and the results were near miraculous: it improved their insulin resistance and prediabetic state in many key metabolic pathways. 
Again, this is incredibly important because so many middle-aged and beyond adults in a modern lifestyle struggle with M, prediabetes and insulin resistance. BCAA's is likely another potent weapon in their glucose metabolism arsenal. (See my link on Prediabetes Prevention for more information.)
All of this may be why a study of middle-aged mice showed even more impressive gains: a 12 percent average increase in life span from Branched Chain Amino supplementation.  Of course, longevity studies on mice don't always translate into the equivalent gains in humans, but one can't help but see that the current research is showing that BCAA's are likely just as beneficial for the middle-aged - and maybe more - than for young, healthy bodybuilders. The middle-aged may be able to avoid much of the muscle-wasting effects of the latter years by taking just a few grams of this supplement or making sure to include enough protein in the their diet.
1) J. Nutr, 136 (1 Suppl):269S 73S, "Branched-chain amino acids activate key enzymes in protein synthesis after physical exercise"
2) Int J Sport Nutr Exerc Metab, 2010 Jun, 20(3):236-44, "Branched-chain amino acid supplementation before squat exercise and delayed-onset muscle soreness"
3) Med Sci Sports Exerc, 2000 Jul, 32(7):1214-9, "The effect of BCAA supplementation upon the immune response of triathletes"
4) Journal of the International Society of Sports Nutrition, 2009, 6(Suppl 1):P1, "Consuming a supplement containing branched-chain amino acids during a resistance-training program increases lean mass, muscle strength and fat loss"
5) J. Nutr, Dec 15 2010, "Higher Branched-Chain Amino Acid Intake Is Associated with a Lower Prevalence of Being Overweight or Obese in Middle-Aged East Asian and Western Adults"
7) Cell Metab, 2009 Apr, 9(4):311-26, "A branched-chain amino acid-related metabolic signature that differentiates obese and lean humans and contributes to insulin resistance"
8) Diabetes, Jun 2007, 56(6):1647-1654, "Increasing Dietary Leucine Intake Reduces Diet-Induced Obesity and Improves Glucose and Cholesterol Metabolism in Mice via Multimechanisms"
9) PLoS ONE, 2011, 6(6):e21187, "Dietary Leucine - An Environmental Modifier of Insulin Resistance Acting on Multiple Levels of Metabolism"