At the most fundamental level, biology is an applied form of chemistry. Biological systems, like everything else in the universe, are made up of matter, and it is the chemical reactions that that matter undergoes which give rise to biological systems, and life as we know it, at both the organism and ecosystem levels of biology.
This applied form of chemistry is known as biochemistry, and concerns the chemical reactions of biological molecules. Many of these molecules are huge, and have extremely complex chemical reactions. It is these biological macromolecules which are the focus of research in biochemistry.
Biological Macromolecules
There are four major types of biological macromolecule which work together in biological systems.
Proteins - Proteins are the functional machinary in biological systems. The diversity of their structures gives rise to an enormous diversity of function, which includes many functions that are vital to life.
Carbohydrates - Carbohydrates are primarily a water soluble energy source for organisms, containing the elements carbon, hydrogen and oxygen. They also have some specialized roles in other biological systems.
Lipids - Lipids are a type of water insoluble molecule that has many roles in organisms as an energy source, for insulation, and making up the structure of the cells themselves.
Nucleic Acids - Nucleic acids are type of molecule used for information storage and and transmission, most notably in the form of deoxyribonucleic acid, or DNA.