Bash the target with your shield dazing them and interrupting spellcasting, which prevents any spell in that school from being cast for 6 sec.
Shield Bash is a warrior skill in the Protection tree. It requires a shield to be equipped and is capable of interrupting spellcasting. If a spell is interrupted, all spells in that school are locked for 6 seconds. Shield Bash also dazes the target.
Shield Bash has the advantage of generating a lot of threat, almost as much as Sunder Armor (Sunder Armor generates 301 threat at its maximum rank, Shield Bash generates 293 threat at its maximum rank). Unlike Shield Slam, it should be noted that the damage component of this attack is not mitigated by armor. The rank 4 skill will do 63 damage so long as the attack hits the target. (The attack is subject to normal avoidance rules, so it can miss, or be dodged etc.) This attack can also crit, in which case the damage will double to 126.
As of patch 3.0.2, Shield Bash no longer causes damage. It can still miss or be dodged, parried or blocked. It is unknown if this has affected threat values.
Shield Bash can be useful while tanking. Interrupting a spellcasting mob usually results in them switching to melee attacks, which generally cause far less damage and also allows you to pull them back, to avoid patrolling mobs. Many raid mobs are immune to the daze effect (these are often the same mobs that are immune to the silence effect). A wise use of this skill will include the use of Heroic Strike (Rank 10), which will add an additional 62 damage to dazed targets.
Shield bash is the first spell-interrupt warriors have (Pummel being the other one). Its use is therefore limited to spellcasters while the warrior has a shield equipped. That said, it is highly useful against casting mobs and should, with few exceptions, be used each time the mob is channeling a spell.
Shield bash will also interrupt most spell-like attacks like venom spits and life or mana leech.