The spell or spells that can counter the specified spell in addition to the universal counters and itself.
How Counterspells Work
To use a counterspell, the character must select an opponent as the target of the counterspell. The character does this by choosing the ready action. In doing so, the character elects to wait to complete his or her action until the character's opponent tries to cast a spell. (The character may still move at normal speed, since ready is a standard action.)
If the target of the character's counterspell tries to cast a spell, the character makes a Spellcraft check (DC 15 + the spell's level). This check is a free action. If the check succeeds, the character correctly identifies the opponent's spell and can attempt to counter it. (If the check fails, the character can't do either of these things.) To complete the action, the character must cast the correct spell. As a general rule, a spell can only counter itself. If the character is able to cast the same spell and has it prepared (if the character prepares spells), the character casts it, altering it slightly to create a counterspell effect. If the target is within range, both spells automatically negate each other with no other results.
Counterspelling Metamagic Spells
Metamagic feats are not taken into account when determining whether a spell can be countered.
Some spells specifically counter each other, especially when they have diametrically opposed effects.
Dispel Magic as a Counterspell
The character can use dispel magic to counterspell another spellcaster, and the character doesn't need to identify the spell he or she is casting. However, dispel magic doesn't always work as a counterspell.