The structure of protein molecules describe ?

Proteins are polymers composed of repeating units of amino acids, linked via peptide bonds (bonds between the amine and carboxyl groups of the adjacent amino acids). All proteins have a primary, secondary and tertiary structure and some, such as haemoglobin, have a quaternary structure. 

Primary structure of the proteins are the sequence of amino acids and their order. The "R" regions of the amino acids determine the proteins secondary tertiary and quaternary structures.

In the secondary structure, the protein folds into either an alpha helix or a beta pleated sheet. This occurs due to hydrogen bonding between the "R" group of the amino acids.

The tertiary structure gives the protein its 3D shape. Here it is folded further and more bonds (such as disulphide bonds) also form.

In the quaternary structure, prosthetic groups (e.g. a haem group for haemoglobin) is added. If the protein has more than one protein chains, here the chains join to form the final protein.