Proteins have a lot of uses in the body. from cell structure to carrying out reactions. Proteins are in turn made of smaller building blocks called amino acids. There are a total of 20 amino acids.
Proteins can be made from a lots of different combinations and number of amino acids. This creates an almost infinite amount of different proteins. Although the sequence of amino acids is a chain, the structure of proteins is far from similar. These amino acids interact with each other to form #D structures. Most common are the alpha helix and the beta strand.
Although it is possible to determine the 3D structure of a protein, it is not quite possible to predict protein structure from the amino acid chain. This is quite important because diseases such as Alzheimer's and Mad Cow Disease are thought to come from a misfolding of proteins. 
- The Structures of Life, NIH Publication No. 07-2778