Amino Acids are the building blocks of all the proteins found in nature. The general structure shared by most amino acids is a central carbon atom referred to as "alpha carbon." Attached to this carbon is an amine (-NH2) group and a carboxylic acid (-COOH) group. The third group attached to the alpha carbon is a hydrogen atom and the fourth group is generically listed as "R." This fourth group varies for each amino acid and is grouped by the following characteristics: (1) polar, (2) non-polar, (3) acidic, and (4) basic.
An excellent guide showing and classifying all twenty amino acids can be found here.