The Ogham is a Celtic pre-Christian alphabet which was carved into stone and wood, with the letters being carved notches from/across a central line (generally read from bottom-to-top).
Due to the associations between the letters and various trees, Robert Graves published a book in 1948 turning the fews (letters) into a divinatory system, which has since been expanded on.
The original alphabet was made up of 20 fews (5 fews each in four groups, or aicme), with an additional 5 (the Forfedha) added much later.
The Letters of the Ogham
*-Nb: In some modern practices, Fearn and Nuin are swapped. I have chosen to go with this order/translation, but both orders are commonly used.