Horn of Winter is a death knight ability learned at level 65. It is a group buff that increases attack power. The ability also generates a small amount of runic power when used. The buff should be kept up at all times, as the attack power gained will increase your damage done.
Horn of Winter should be used before every pull for the free runic power. It should also be used any time you have a spare global cooldown to generate some extra runic power and to make sure the buff does not fall off.
Patch 3.2.0 (04-Aug-2009): Learning rank 2 of this ability caused players to unlearn rank 1, such that it no longer appears in their spellbook. It is now possible to relearn that rank, and death knights will no longer "forget" rank 1 when they learn rank 2.