Proteins are composed of long sequences of amino acids. Amino acids are composed of a carboxylic acid, an amine, and a side chain group that defines the nature of the amino acid. The carboxylic acids and amines of differnt amino acids combine to form long chains of amino acids linked together by amide bonds. The various sequence of amino acids used and the exact order in which they are put together will determine exactly which protein you end up with. The sequence of amino acids will determine the shape of the different subunits of the protein (alpha helix or beta sheet) and the way the different subunits fold together will determine the shape and ultimately the chemical function of the protein. Keep in mind that there are thousands of different proteins found in nature but there are only 20 or so commonly found amino acids. So from a relatively small number of amino acids we can get a huge variety of proteins based on exactly how they are put together.