Protein is made up of amino acids but not all amino acids are created equal. Branched Chain Amino Acids (BCAAs) are the most popular amino acids in sports nutrition due to their impressive effect on muscle growth and recovery. These are Leucine, Isoleucine and Valine and can be found in both capsule and powder form. We'd recommend a flavoured powder if unsure because you have to take 6 capsules at a time for most products, and plain powder tastes very bitter if not flavoured.
A new development in amino acid supplementation is custom amino acid blends; this type of product is where a brand "cherry picks" the amino acids they think are most important and create a product with their own "perfect" blend - for example combining the BCAAs with Glutamine makes for a great recovery product.
Another type of amino acid supplement is Essential Amino Acid (EAA) products. These products contain ONLY the amino acids that your body cannot make on it's own - this makes sense as you are only paying for the amino acids which you really need to get through diet and supplementation.
Amino acid supplements also tend to be very low (or even zero) calorie making them perfect for both protecting muscle when dieting hard and also for a feeling of fullness when dieting.