The Taunka reputation is one of 4 reputations added in Wrath of the Lich King under the sub bracket "Horde Expedition". The Taunka are a race very similar to the Tauren. They are infact, Ancestral Cousins to the Tauren. They appear in a few different places through out Northrend but you could call their home "hub" to be Taunka'Le Village in Borean Tundra.
There are two ways for players to raise the Taunka reputation. One of them is to complete all the Taunka quests in Borean Tundra, Dragonblight and Storm Peaks. The other way to raise your reputation (added with patch 3.3) is to not wear a faction specific tabard while running heroics.
I'd like to also note, to prevent confusion. It will show your Horde Expedition faction and your Sunreavers Faction increasing but not The Taunka faction. If your Horde Expedition faction and Sunreavers faction is exalted then it will display your Taunka faction as increasing. However if you're not exalted with Sunreavers and the Horde Expedition, you will still gain Taunka faction.
It's confusing I know. But you'll understand once you see it happening.