Branched Chain Amino Acids (BCAAs) are Leucine, Isoleucine and Valine.  These three essential (meaning the body cannot manufacture and thus must ingest from foods) amino acids make up approximately 1/3 of skeletal muscles in the human body, and play an important role in protein synthesis.  Studies have shown that they help preserve glycogen (energy) in muscles and prevent muscle tissue breakdown during and after training.  Unlike other amino acids, BCAAs are not metabolised in the liver and are instead metabolised directly in the muscles.  This unique quality means that they are very anabolic (they build muscles) and very anti-catabolic (they stop muscle breakdown).