An amino acid is a biologically important molecule in that it is the basic building block of all proteins. Amino acids consist of a central carbon atom that is bonded to a carboxylic acid, an amine, a hydrogen, and a side chain group. The nature of these side chain groups is what distinguishes all of the amino acids from one another and gives them their characteristics.
There are literally hundreds of potential amino acids but 22 of them are the most commonly found ones in nature that are used to build naturally-occurring proteins (said to be proteinogenic). 9 of these 22 are called essential amino acids since that cannot be chemically produced in the body and must be obtained from the consumption of food or supplements.