Youjutsu (lit. "Demon Arts") is a technique used in High School DxD.
Youjutsu is the Youkai form of spellcraft and is mainly unique to the Youkai. Currently, Kuroka and Sun Wukong are the only Youkai that have used Youjutsu, while Koneko is still training on how to to properly use it.
Youjutsu is an ability unique only to Youkai that allows them to cast both offensive and defensive spells.
Other spells include creating and controlling illusions and producing a poisonous mist that's only effective against specific races such as Youkais and Devils.