Why is DNA important?

DNA, or deoxyribonucleic acid, is important as a genetic code in a living organism.
Humans are not the only creatures with DNA. Plants have DNA, animals have DNA, and even fungi and bacteria have DNA. 
DNA as a molecule is really a code that the cell can read, containing all of the information that governs the body. Cells can read this code to determine the structures the cell will build and destroy, and how long the cell should wait between different actions.
Mutations are really changes in the code of an organism's DNA. When DNA is altered, the cells will change how they are built.
DNA is the most likely originator of viruses, and is used by viruses to replicate.
DNA is especially important in the study of genetics and medicine, as it can be responsible for many ailments.