Ogham is an alphabet that was primarily used to write Old Irish, around the 4th century AD, to the 10th century. Sometimes it is referred to as the Beth-Luis-Nion, either meaning the Beth-Luis letters, or a shortening of the first grouping of letters. In myth the invention of the script is usually attributed to Ogma, a member of the Tuatha Dé Danann.
Originally the alphabet was made up of twenty letters, or feda (trees), grouped in four family groups, or acimi (the plural for aicme, or family). Later five additional letters were added, the forfeda. Each letter has a meaning, most commonly each is associated with a tree. However, it seems the original associations were not so cut and dry. Some sources say only 7 of the letters can faithfully be associated with plants - other sources give a few more, but not all. The other letters are given meanings such as earth, gold, field, sulfur, and fear. I would strongly encourage anyone interested in Ogham to look at the historical information (from good, historical sources), and consider the other meanings for each.
The letters are often used for divination. There is, as far as I know, only one historical reference to the letters being used this way. It's quite vague, so one wishing to use the Ogham for divination must form their own system. How many letters will be set down, if a spread will be used (like a card spread), or if they will be thrown down on a cloth, etc. Additionally, the letters can be used in meditation and spellwork, like other symbols.