Biochemistry is about the study of the organic molecules that make up living systems (called biomolecules) and the chemistry that they undergo in a living organism to sustain life.  Since biomolecules tend to be very large and complex, they have been divided into four main groups: amino acids, nucleic acids, lipids, and sugars.  Amino acids are the building blocks of proteins and catalysts.  Nucleic acids are the building blocks of DNA and RNA.  Lipids are fats and make up things like fatty tissues and cell membranes.  Finally, sugars are used for everything from food and energy sources to structural support.  They can range from simple food sugars to complex starch polymers.