This document discusses the structure and classification of amino acids. It begins by stating that amino acids are the basic structural units of proteins and that there are 20 standard amino acids found in protein structures. The general structure of an amino acid is presented, showing that they contain an amino group, a carboxyl group, and a variable R group (side chain) attached to the alpha carbon. Amino acids are then classified into four types based on the polarity of their R groups: non-polar aliphatic, non-polar aromatic, polar uncharged, and polar charged (acidic or basic). Examples of amino acids that fall into each of these four categories are provided.