Amino acids are the building blocks of protein, all proteins contain amino acids it is just a case of whether they are full spectrum (contain all amino acids), essential amino acids or leucine dominant amino acids. Amino acid supplements generally come as both amino acid tablets and amino acid powders.
Many amino acid supplements tend to focus on branched chain amino acids (BCAAs), of which there are three: leucine, isoleucine and valine. Leucine is the most important BCAA for muscle growth as it can effect the mTOR anabolic pathways. Most BCAA formulas have a 2:1:1 ratio of leucine to isoleucine and valine.
Often amino acid powders are taken during the workout to stop catabolism (muscle waste) and aid recovery. Amino acid tablets tend to be taken at any time through the day whenever a boost in amino pools is needed. Another good time to use amino acids is when you are dieting and trying to lose body fat, taking amino acids prior to doing fasted cardio can stop catabolism while still allowing you to tap into fat stores.
As pointed out above, the best amino acids for muscle building are leucine, isoleucine and valine, when coupled with other supplements such as HMB it will aid the reduction of catabolism and improve recovery, an ideal example of one of these products is NXT Nutrition Amino Fuel which is a perfect intra workout supplement.