DNA is essentially the set of instructions that create and direct everything biological.

Our bodies are made up of trillions of cells grouped together. Within each cell is a complete strand of DNA. DNA is essentially the set of instructions that create and direct everything biological. These instructions tell our cells how to grow, work, divide, and die.

Our bodies are growing new cells and biomolecules (hair, teeth, bone, etc) all the time. To do this the DNA in our genes provide the instructions to our stem cells which then create the specific types of cell we need. It's important that we understand the role stem cells play in our bodies, because it’s now widely accepted that all cancers are caused by adult stem cells (or cells with stem-cell like properties). 

From Alex:

Our bodies are made of many different components. Our bodies know how to make these components because of the code (signaling molecules) within our DNA. DNA is a double helix chain of amino acids, which are the building blocks of proteins, attached to a sugar. The order of these amino acids creates the code for a new type of a protein or enzyme our bodies need. This could be a protein in our hair, skin, or a component of our blood.

DNA is composed of four different amino acids: Adenine (A), Thymine (T), Guanine (G) and Cytosine (C). Adenine and thymine connect and guanine and cytosine connect. They always connect in this pairing. It's the order, or sequence, of these pairs that determines what is created.

These pairs link two strands and create a double helix. One strand is the coding strand, the other is the template strand. Our bodies create the necessary protein or enzyme by reading the coding strand. The template strand is a redundancy that is used to repair any damage that occurs to the coding strand. 

What is interesting is that genes and other transcribed elements can be encoded on either strand in either direction. So in other words genes can be inside of genes in different orientations. (what we now know is that some genes can be on the coding strand backwards, they can be in any orientation and any position. Like a crossword puzzle the words can be in any orientation and still understood by our bodies.) This adds to the complexity of how our DNA can be used.