This build is necessary to use for one mage in a raid to increase the damage of all the other mages!
Indeed, this build is centered around the Winter's Chill talent which increases the critical strike chance of all Frost spells by 10%. It does less damage than the Arcane Power Frost Build, but the buff is essential to increase the damage of all the other mages in the raid.
This build is the best raiding and dungeon build before the Ahn'Qiraj availability in phase 5 simply because a lot of bosses and monsters in Molten Core and Blackwing Lair are immune to Fire spells, so Frost ones are logically most effective.
Basically, it consists in using continuously the Frostbolt spell, which is the best single target DPS frost spell available to mages. Because the Frostbolt spell is also very mana efficient, you can freely focus on damage gear rather than mana regeneration one.
However, while Fire spells are supposed to do more damage, keep in mind that they also cost more mana, so make sure to have a proper mana regeneration gear before using this build. Also, the Ignite talent is very powerful but must be coupled with gear containing stats which improve your critical strike chances.