Amino acids universally contain an acidic carboxylic acid functional group connected to their alpha carbon. The alpha carbon is also connected to an amine functional group and an R group that is distinct to each amino acid.
Amino acids are so named due to the amine and carboxylic acid functional groups they contain. There are 20 common amino acids, with each being distinct in its R group side chain. The most simple amino acid, glycine, contains a single hydrogen as its R group.
Amino acids are commonly sorted into three categories: nonpolar, polar uncharged and polar charged. The polar charged amino acids can be further divided into acidic or basic amino acids. The two acidic amino acids, aspartic acid and glutamic acid, both contain an aditional carboxylic acid functional group at the end of their R group.