There are two things referred to as the Witches; Runes and the Witches' Rune.
The Witches' Rune is actually a chant written by Doreen Valiente as part of Wiccan practice. You can find it at http://www.sacred-texts.com/pag/gbos/gbos36.htm
The Witches' Runes are a modern divination system. They are in no way related to the runes used by the Norse and Germanic cultures. Unlike the Elder and Younger Futhark where the runes represent letters of the alphabet as well as acting as divinitory tools, the Witches' Runes are a series of 13 symbols that can be used for divination but don't really have any other use. You can find pictures of them with their meanings at http://www.tarotwikipedia.com/the-13-witches-runes/ Frankly I've never met any Witches who use this system.
I have also heard the Theban Alphabet called Witches' Runes, but it isn't that at all. It is simply an alphabet that some use to keep material in their Book of Shadows from prying eyes.