In order to successfully counterspell, you must first detect the spell your offensive target is casting, which is dependent upon your skill in Spell Identification. If detected, an audio cue will sound to indicate that your counterspell is available to use, and you must then activate the ability before the opponent completes their cast. Your skill in Counterspelling improves the chances your interrupt will succeed.
The basic counterspells obtained at level 10 simply interrupt the cast, causing the refresh timer of your offensive target's spell to begin even though the effect has been cancelled. Most classes receive an improved counterspell at level 30 however, which may provide secondary benefits to yourself or your group after each successful use.
Examples of counterspell abilities include: