Scrolls are a specific kind of magic item. A magic scroll contains an enchantment that is released when it is chanted by a wizard. Only a wizard may use a scroll. A scroll may only be used once, so when it has been used it is useless.
The two scrolls included with the Warhammer game contain the powerful counterspells Dispel and Destroy. Both are used to counter the effect of enemy magic, and are used to interrupt the normal spell casting sequence. If the enemy casts a spell and you wish to counter it then you must immediately declare you are using the Dispel or Destroy scroll. The enemy's spell is stopped until the results of the counterspell are resolved.
The Dispel scroll can be used to dispel any spell regardless of where it is cast or at whom. The Destroy scroll can only be used if the wizard carrying it is directly affected by the spell, ie if it is cast either at him or at the unit he is with.
Note that we have provided only one each of these scrolls, as to include more would have reduced the number of other magic items. If you wish to copy these scrolls to make more for your Warhammer games then please do so, there is no reason why a wizard shouldn't carry several scrolls. The total number of magic items a character can carry is limited as described in the section on Heroes and Wizards. Even the greatest wizard cannot carry more than 4 magic items in total, so it will always be a tough decision whether to carry scrolls or other items.