Living matter is built out of a large range on elements, all of which are vital to all earthly life.

However, only four of these, Carbon, Hydrogen, Oxygen and Nitrogen, make up the majority of all organic matter on this planet, over 95% of it. Phosphorus is next most common.

Life on other worlds may not look like us or use DNA, but on any other world (except those so utterly unlike earth that we have no idea how life could form at all), these elements are an excellent bet to make up any kind of life.

The concept was mentioned in Frederick Pohl's Gateway Science Fiction books, though I have no idea if he invented the word CHON.

Carbon is the skeleton, the backbone and the rings of the long organic molecules. Carbon is the single most important element for life, because it can bond to three or four other atoms, or double up it's bonds with other carbon atoms, and thus form chains, rings and branches.

Oxygen and Nitrogen are the modifiers, they put kinks and twists in the carbon chains and have two bonds.

Hydrogen, with it's single bond, caps the ends of the chains.

All of these are available in air or water: Oxygen and Nitrogen as biatomic pairs, O2 and N2, Carbon Dioxide for carbon in the air, and water (dihydrogen monoxide) for hydrogen and more oxygen.

From these four elements you can make everything from sugars and starches to petroleum oils and simple proteins.

The Alchemist points out that the human body composition (by weight) is:
61% Oxygen
23% Carbon
10% Hydrogen. This is low as hydrogen atoms are the lightest.
2% Nitrogen
Making 96% total. Next up, the only other two with more than 1% are Calcium at 1.1% and Phosphorus at 1.1% See the node for the rest.
