Spellcraft is used to identify spells and to perform counterspells.
Check: A successful check means that the character has identified a spell being cast by an opponent. The DC of this check is equal to 15 + spell level. The character also gains a +1 bonus for every 5 ranks in this skill to all saving throws against spells.
Special: A specialist wizard gets a +2 bonus when dealing with a spell from his specialized school. As well, a successful Spellcraft check is required before your character can attempt to counterspell. The specialist wizard suffers a -5 penalty when dealing with a spell or effect from a prohibited school.