3.0: Page 203 of the DMG goes into a detailed description of what is required to be able to activate a scroll. Assuming the character has decyphered the scroll with a Read Magic spell or Spellcraft check (DC 15+spell level), the other 3 major requirements are:
1) The spell must be of the correct type (arcane or divine). I take this to mean that if a 1st level cleric scribed Cause Fear, a 1st level wizard would be unable to activate the scroll, even though Cause Fear is a 1st level wizard spell.
2) The character must have the spell on their class spell list.
3) The character must have the requisite ability score to cast spells of the given level.
Note that any spellcasting class (Bard, Cleric, Paladin, Ranger, Sorcerer, and Wizard) can use scrolls, or craft them if they have Scribe Scroll. You can use a scroll crafted by somebody of a different class as long as you still meet all the criteria above.
Use Magic Device basically allows the Rogue/Bard to emulate either a class feature of another class or a different alignment when attempting to activate a magical device. It is a very powerful skill, since it applies to any and all magic items that are in one way or another restricted to a particular class or alignment. A Neutral or Evil rogue could use this skill to activate a Horn of Goodness/Evil to create a Magic Circle Against Evil, for instance, when normally the Neutral rogue couldn't activate it at all, and the Evil rogue would activate a Magic Circle Against Good. Or he could wield a weapon that would otherwise deal damage to somebody of his alignment. UMD is most often used for wands and scrolls, but it is considereably more versitile.
