The tooltip for counterspell states:
"Counters the enemy's spellcast, preventing any spell from that school of magic from being cast for 10 sec. Generates a high amount of threat"
Testing on the Defias Pillager's in Moonbrook. When counterspell is cast, while the Pillager is in the process of casting a fireball, it does interrupt that cast, but they start casting another fireball within 1 second.