A Scroll is a magic item. It is a physical storage device for a single Spell, which can be used only by a Caster. Casters may create a scroll or scrolls on their Turn, for a significantly higher cost in juice than simply casting the same Spell.
Even a Caster who is normally nor proficient at casting spells beyond his own discipline can use scrolls of other disciplines with some effectiveness.
It is not clear whether casting a Spell from a scroll spends any of Caster's juice.