Amino acids are composed of a carbon atom, a hydrogen atom, a carboxyl group along an amino group, and a variable group called the “R” group. The variable "R" group is what distinguishes the different amino acid categories. Amino acids can be classified into four categories – polar, nonpolar, positively charged, or negatively charged - depending on the properties of the “R” group in each amino acid.

• Polar amino acids have a hydrophilic “R” group that seeks contact with water and aqueous solutions. Cysteine, Glutamine, and Asparagine are some amino acids that come under the polar category.
• Nonpolar amino acids have a hydrophobic “R” group and avoid contact with water and other liquids. Glycine, Tryptophan, and Alanine are some amino acids that come under the nonpolar category.
• Positively charged amino acids are also called polar basic amino acids. Arginine, Histidine, and Lysine come under the category of positively charged amino acids. 
• Negatively charged amino acids are also called polar acidic amino acids. Glutamate and Aspartate are negatively charged amino acids.