Spell Reflection is a core warrior ability learned at level 66. It is a self-buff that allows the warrior to reflect a spell back onto the enemy caster. It lasts for 5 seconds, and can be used both offensively and defensively.
Some "boss" mobs in general and a lot of mobs in raids are immune to their own spell, but reflecting it means you don't suffer its effects, such as Fel Fireball against Lord Jaraxxus in Trial of the Crusader.
Something less known about Spell Reflection is its ability to reflect multiple spells if they are both cast while the buff is still active (i.e. leave the caster's hand). This makes it very useful when coupled with a charge, as it allows the warrior to reflect the initial spell from each casting mob simultaneously if their casts have the same length and start at the same time.
Patch 5.3.0 (21-May-2013): Spell Reflection is no longer consumed by spells from player pets and guardians.
Patch 4.1.0 (26-Apr-2011): Spell Reflection cooldown has been increased to 25 seconds, up from 10.
Patch 3.3.3 (2010-03-23): Using this ability to reflect a Mind Flay now always properly consumes the Spell Reflection buff.
Patch 2.3.0 (13-Nov-2007): Spell Reflection no longer has a global cooldown.
Patch 2.1.0 (22-May-2007): Reflected spells generates threat as if the damage were dealt by a normal attack. Also, this threat is subject to the usual threat multipliers granted by Defensive Stance and/or Defiance. This change was not officially noted, but it has been confirmed by several players.