Runes are magical symbols of various sorts that are used as a focus to form spell effects. There are several types of runes in use. The symbols themselves have no inherent power without the mana that was channeled into them to activate them.
The primary rune system used by the people of the planes is simply known as the runes of magic, due in part to ignorance of the Tinean runes. There are seven thousand runes stated, although that number is approximate, as most people do not know anywhere near that number, and a clever mage can create new runes for their own use.
The runes in Torn Elkandu and on other Nexi are Tinean runes, derived from Til'dari runes. They have a distinctive, interlocking triangular shape, unlike other rune systems which tend to be based around squares and linear writing.