What are Proteins and Why they are important for us?

We just read that proteins are essential for maintaining and sustaining life. What are proteins or what constitutes proteins that make it so important for us? Proteins contain carbon, hydrogen, nitrogen, sulphur and some also contain phosphorus. You may recall studying that proteins are chains of amino acids held together by peptide bonds, when a peptide chain is extended by more and more amino acids, until a chain length of one hundred to several thousand amino acid residues is reached, it is classified as a 'protein'. The nitrogen content of proteins varies from about 14 to 20 percent. The average value of 16 percent is used commonly for converting nitrogen content of food stuffs or tissues into proteins by multiplying with the factor 6.25 (100116) (i.e. Crude protein = Nitrogen x 6.25).