Spell Scrolls are another way to cast spells. They have a number of very specific instructions on their use.
First, only a magic casting class can use them.
Second a caster can only cast spells from their spell list (Though I would also rule that they can also cast spells they have access to some other way – ala feat, etc)
Third, the spell must be of a level the caster can already cast. If they want to cast a higher level spell, they can, but must roll an ability check with their spellcasting modifier.
Finally – if a spell is cast successfully, or copied, or cast unsuccessfully, the spell disappears — you only get one shot!