A steam tonk is a controllable toy tank found at the Darkmoon Faire.
About
The new (as of patch 1.10) dwarven-made Steam Tonks of the Darkmoon Faire are a huge success. Players can get to a control panel anywhere in the Faire, and control a little steam-tank-like tonk. With this Steam Tonk, you can only attack other tonks, not players. In short, tonks are a cross between the safety of a duel and the multiplayer madness of a battleground. Note: They dismiss your pet when used.
Players with an Engineering skill of 275 can obtain the schematic for a [Steam Tonk Controller] from the Darkmoon Faire. The schematic costs forty Darkmoon Faire Prize Tickets. As of patch 2.3, it is now sold in Stormwind also. A Steam Tonk Controller can be used by anyone — it is not bound to engineers only. A Steam Tonk Controller has fifty charges and requires 3x 
[Mithril Bar] and 1x 
[Gold Power Core]. The Steam Tonk Controller will summon a Steam Tonk pet that remains active for five minutes or until the Steam Tonk is destroyed. Steam Tonks can be destroyed by other Steam Tonks and by going outside of the range of the Steam Tonk Controller. A summoned Steam Tonk can not engage a hostile NPC in battle. They can be summoned on sea-going vessels and inside instances.
Non-engineers can get a [Battered Steam Tonk Controller] from Quest:Missing Friends in Terokkar Forest.
When
Tonks are available for use during the Darkmoon Faire at Thunder Bluff, Goldshire or Shattrath City during their respective months.
How
Tonks are controlled using your movement keys (default or custom) and using the extra spell bar, similar to hunters pets or warlocks minions. All tonks have 2 basic attacks: The cannon, which shoots straight foward for 20-27 damage and the mortar which does 43-48 damage but has a very long range. All tonks also have a Nitro Boost, which increases speed at the cost of mana. In addition, each Tonk also has a special ability like 
[Mana Shield] (absorbs damage at the cost of mana), Flamethrower (consistant damage to anything in front of the tonk), Machine Gun (regular 3-6 damage to a single nearby tonk) and Tonk Mine (plants a mine that does heavy damage to any tonk that runs over it). They can also jump. As of late however, tonks only seem to have the Tonk Mine ability as their special. Tonks do not aggro hostile NPC's, and can be used to "scout ahead" to a target and let you put raid icons on mobs that are not in your line of sight.