Briefly, ogham (pronounced OHM) is the carving alphabet of the Irish (later adopted by the Welsh). It was typically used to carve into stone (the line we see would be the edge of a stone) or likely wood (which, naturally hasn't survived).

There are a number of good sites on ogham, its history and use, and so I will not recount that here. However, I will show a small project I was working on, that is, creating an ASCII ogham alphabet. It occured to me that such an alphabet, given its nature of straight and slanted lines, would work quite well in ASCII (and thus be readable all over the internet and not require the downloading of special fonts). This is what I have devised:

   b l	 f   s     n 

   h d   t   c     q  

  m  g  ng   st    r  

   a o   u   e     i

  ch   th   ph  pe  xi
  ea   oi   io  ui  ae

As you can see, each letter is ably expressed through ASCII means, until we reach the final five, the so-called "extra letters" of the alphabet. In this case, the alphabet is usually depicted thus:

However, this is not fully presentable through ASCII, and so the Livejournal user Straif devised the scheme above for the dipthongs. I think they work rather well.

Every Ogham Thing on the Web
The Ogham Tract from the Book of Ballymote
Ogham: Upon Reflection