All amino acids have a primary amino group (-NH2)
and a carboxyl group (-COOH) linked to the same carbon atom (this is called alpha carbon, because it is adjacent to the carboxyl):
The letter "R" represents the rest of the molecule, and it is called the sidechain of the amino acid; it's what differentiates one amino acid from the others.
At physiological pH, the amino group is protonated (-NH3+)
and the carboxyl group is not (-COO−).
Proteins are composed of 20 types of amino acids, called natural or canonical. These 20 amino acids differ from each other in the structure of the R sidechain. According to the nature of this sidechain, we can classify amino acids into 5 groups: