Ogam or ogham is an ancient alphabet, based on staves. It is sometimes referred to as "runes", but this is incorrect, and should refer to the Norse alphabet instead.
The name appears to be of Old Irish origin, but in Shetland its use is associated with the Picts. Shetland appears to represent the most northern part of ogham's range - the most southerly is around the area of Devon and Cornwall. The majority tend to be found in Ireland and Wales. Ogham is also found in Orkney, but appears to be unknown from the Faroes and Norway.
The most famous instances of ogham in Shetland are to be found on the Bressay Stone.
The use of ogham is extinct, although some neo-pagans try to use it for their purposes.