Permanent runes are branded onto blades to produce silver swords of a higher quality than the base weapon, they can not be used on steel swords. The permanent rune stones are combined in threes.
There are also temporary runes, which Geralt can apply himself. These have blade enhancing effects similar to oils, and cannot be used at the same time as an oil on a given blade. Temporary runes can be applied to either steel or silver swords and provide a temporary upgrade to the weapon, similar to weapon oil in duration. The enhancement depends on the rune stone used, but all last for a period of 24 hours.