When you successfully block it reduces the damage by about half, and, on non-mage characters it allows you to do a counter attack.

Unfortunately, at higher levels with everyone doing almost continuous skill attacks block is useless. Most higher level bosses also ignore blocking, or sure seem to, I haven't been able to block on in ages.

Rune is a bit useful against some Heroic bosses minions though. Helps reduce their damage, gives you a shot back at them, and gives you longer to kill them.