DNA or Deoxyribonucleic acid is the acid found in the nucleus of a cell, which contains the basic genetic instructions for the cell, e.g. how to grow, how to reproduce, etc. The only living being to be without DNA is an RNA virus.
It is considered to be the "blueprint" of a cell, as it also contains the ultimate plan of how the cell will function, look, and what purpose it will fulfill. According to the theory of evolution, every time that a new offspring is conceived, the DNA of the new organism is ever-so-slightly changed, creating a new type of life form, but still very similar to its predecessor. The new genetic code gives a different order, which, say, will tell the cell to activate its lysosome under another condition, allowing for advancement in the life form. DNA is a particle that comes in the form of a double-helix, constructed by molecules and some small clusters of atoms, which create the combination to the genetic code.
DNA plays an extremely important role on not just about.... everything. It can determine how tall you'll be, how easily you can build muscle, how fast or slow your metabolism is, how well your immune system is, your natural skin tone, natural hair color, eye color, ect. and new studies show it can even determine some personality traits of the organism.