All amino acids have a carboxyl group and an amino group bonded to the same carbon atom (which is the α carbon). However, they differ from each other in terms of their R group or the side chain group. The nature of the R-group differentiate each amino acid in terms of their acidic or basic nature, solubility, stability, and ultimately the proteins.
20 such amino acids, differing in their side chain and distinctive chemical and physical properties have been identified. Proteins, of all species from ancient times, are made out of these same ubiquitous set of 20 amino acids. Thus these group of 20 precursor amino acid molecules are like alphabet for composing protein molecule.