Just thought I'd chime in one last peice of info not mentioned yet: Scrolls are either Arcane, or Devine, depnding on the scroll's creator. Arcane spell casters can't use divne scrolls, and vise versa (unles the're multiclassed to have acess to both or the Use Magic Device skill, of course). This limitation does not apply to wands or Staves. So Bards can not use clericly scribed scrolls of Cure Light Wounds while, Likewise, Clerics can not use a wizzard's Summon Monster V scroll.
As far as level: for spell completion (scroll) you make a caster level check, 1d20 + your spellcaster level (you need to, of course, use the spell casting class which allows you to activate this spell trigger item. Or use your effective level from your Use Magic Devices check).
Also using a spell trigger item (Wand / Staff) does not provoke an attack of pooertunity, while using a spell completion item (Scroll) does.