A twisted ladder
DNA is a molecule shaped like a ladder that has been twisted into a spiral: the double helix. The two rails are long chains of sugar and phosphate, the same all the way along. The rungs are the interesting part. Each rung is a pair of smaller molecules called bases, one attached to each rail, meeting in the middle.
Four letters
There are only four bases, usually written by their initials: A, T, G and C. They pair by a strict rule. A always sits opposite T, and G always sits opposite C. A rung is never A with G, or T with C.
That rule is the whole secret of the molecule. Read the letters down one rail and you have a message. Read the other rail and you have the same message in mirror form, because every letter fixes its partner. A stretch of DNA reading ATGGC on one side must read TACCG on the other.
The two kinds of pair are not quite equal. An A–T pair holds together with two hydrogen bonds; a G–C pair with three. Stretches rich in G and C are harder to pull apart, which matters to the machinery that has to open the helix.
Try it: choose GC-rich, then highlight G–C. Count the rungs that light up. Now choose AT-rich and do the same.
Unzip and copy
Before a cell divides, it has to copy its DNA. The trick follows straight from the pairing rule. An enzyme unzips the helix down the middle, separating the rungs so that each rail carries a row of unpaired bases. Each rail is then used as a template: free bases floating in the cell attach to their partners, A to T and G to C, and a new rail is built along each old one.
The result is two helices where there was one, and each of them is half old and half new. Scrub Double helix to Copied to watch it happen: the strands separate from one end, and a new partner strand grows on each.
From letters to traits
A gene is a stretch of the message, often thousands of letters long, that tells the cell how to build one protein. Proteins do nearly everything in a cell: they form its structure, speed up its chemistry, carry its signals. The order of the letters in a gene sets the order of the building blocks in its protein, and that in turn shapes what the protein can do.
Your DNA is packed into forty-six bundles called chromosomes, half from each parent. That is why you share traits with both of them, and why siblings, who draw a different half from each parent, are alike without being identical.
Mistakes and variety
Copying three billion letters is not perfect. Occasionally a letter is swapped, dropped or repeated. Most such mutations change nothing noticeable, some are harmful, and a rare few are useful. Over many generations those rare useful changes are the raw material that evolution works with. The same molecule that keeps the message stable also lets it slowly change.