Horn of Winter is a Death Knight skill that was added in a recent build. Much like the Shaman's Strength of Earth Totem, this skill will give a strength and agility buff to nearby party or raid members -- 155 at the highest rank, to be exact.
It has advantages over Strength of Earth to be sure. It lasts 2 minutes like the totem, but unlike the totem, it's quite a bit more mobile, being a straight-up buff. If there's one drawback to the skill, it's finding the time to cast it.
A Death Knight nearly never sits idle while DPSing. Every global cooldown is precious as you strive to use your runes as soon as they refresh and shed runic power as soon as you have enough for your chosen runic power dump. Some Death Knights think that shoehorning Horn of Winter into there might be asking a bit much, and want it to, at the least, not cost any runic power.
Still, it's a very nice "upgraded" version of the Shaman's Strength of Earth totem, and 20 runic power (10 with a glyph) isn't that bad a cost, so if you're in a physical DPS heavy group or raid, and any Shamans aren't putting out that specific totem, it's worth using it. Any slight hiccup in your damage rotation is probably worth all the nice extra AP and melee critical strike rating you'll get from the buff. Just skip a couple Rune Strikes and cast it already!