Runes are the specific letters in a runic alphabet, a form of writing created with the intent that it will be carved into solid surfaces. This gives it a distinctive shape of angles and lines, lacking the curves used in liquid-based writing styles. In the real world, runes are associated with the family of ancient Germanic languages, most notoriously that of the Vikings. Runes were used both for day-to-day writing, and also in certain magical practices, most notably in the form of divination.
In /tg/ media, runes commonly appear as a form of magic. Runes may be the literal alphabet of magic, or may be a methodology by which magic can be bound to physical objects in order to create enchanted items. Dwarves often are depicted using runic alphabets as their writing style of choice, as it's typically presumed that dwarves would rather record lore on stone or metal. Rune-based mages have appeared in various games, most notably the Runepriest.
Runes are sometimes used interchangeably with sigils and glyphs.