TDK -> Wikipedia is fine on this as far as it goes. Which is all almost anyone needs today.
The rest of this limited disclosure is under copyright of George King 02-15-2012 starting here.
"Ogham or Celtic Tree Alphabet a Druid View.
The straight line marking style of writing with an ax or sharp stone served well of writing on Trees and Wooden Poles a free and endless supply of outdoor quality paper for a forest dwelling people.
Of course this is not the true root of of the Ogham language and symbols. Certain ancient and subgroups each took a different tree as identification and informational focal and storage system. From this practice and use by these long gone visitors, their mixed breed decedents continued to use the outward simple aspects of this system for communications. To my knowledge there is few to none that have any real access to its older higher levels of information or encoding. Then there is the always interesting question of the Chicken and the Egg. Did the Ancients adopt the different Tree types or did they bring these seeds with them or alter stock that was already here for their special needs?
Well as nothing is know of them except from (D.B.G.T method) research that few can or chose to do. I will leave it to the reader to discover more. I have peeled back the veil of time just enough to reveal a almost lost trail. Happy journeys to all.
The Druid King"
Copy right 02-15-2012 George King