The following is almost verbatim from De Azcarraga's notes. There is a pile of manuscripts to go through before this section could ever be finished, and if I go through them all, i'll never keep up with my classwork. This too will have to serve as an introduction, possibly to be picked up again and fleshed out at a later time. 

The Ogham script consists of twenty-five simple strokes or
symbols, centred on or branching off a central line. It is similar in purpose,
but separate in origin from the Nordic runes. The Ogham characters were
inscribed on stones or written on staves of wood. As a method of writing it is
laborious, but as a language of symbolism it is powerful. It is probably
pre-Celtic in origin, although most of the existing inscriptions have been
dated to the fifth and sixth centuries. After the 6th century CE, Old
Irish was written with the Roman alphabet, and Ogham disappeared from general
use but the knowledge must have been preserved in some form because our first
knowledge of Ogham comes a 15th-century work The Book of Ballymote.

When inscribed on stones, Ogham is written vertically from
bottom to top. In some cases, mostly in manuscripts, Ogham is written
horizontally from right to left. Sometimes the vowels use dots rather than
lines intersecting the vertical axis.

The Ogham (pronounced owam), or sacred Druidic alphabet, was said
to contain hidden secrets for magic and divination, and only the initiated
could understand these occult meanings. The Celts believed that many trees were
inhabited by spirits or had spirits of their own. They also believed that
certain trees had a healing influence on humans. From this ancient respect for
the power of trees came the expressions 'touch wood' and 'knock on wood'. Oak,
ash, and thorn were called the faery triad of trees. Where they grow together,
it is still said that faeries live.