Hammerhead Gunship
The Hammerhead Gunship is the Tau's analogue to a Tank. Has a distinctly piscine name and can mount railguns.
It can be considered the Tau's "main battle tank", being a general purpose combat vehicle, though its mounted railgun means that it really shines against enemy armor. It has one point less armor in the front than a Leman Russ Battle Tank, but is otherwise similarly fortified. Fluff says that it and the Leman Russ tank are equal. Given the numbers of Leman Russes compared to the number of Hammerheads... probably sucks to be a Hammerhead in the fluff. In the other hand, in Taros campaign Imperial Guard lost five to six Ruses for every Hammerhead destroyed, though waging war in the flat open desert with no cover, under constant markerlight-guided missile barrage and with supply lines being cut almost the second they established may influence that statistics a lot. Unlike the Leman Russ, the Hammerhead is not intended to smash its way into enemy lines, instead hanging back and exploiting the 72" range of its primary weapon to put holes in the Tau's enemies while taunting them to try and rush it. It can be equipped with a disruption pod that gives it a 6+ cover save in the open and 3+ in ruins/fortifications or on the Jink that can help it remain much more survivable than its armor would suggest.
It can take an ion cannon in place of its railgun for the same points value, but without the +5 for the submunition shot, which will do some hefty damage to heavy infantry and monstrous creatures. Forge World makes some additional primary weapon options, which are all pretty good good, with one major exception *cough*missilepod*unconvincingcough*. Secondarily it is equipped with either a pair of independently targeting gun drones, or a single smart missile system. The gun drones have a shorter range and pinning, but the smart missile system can hit from a slightly greater distance and ignores cover and line of sight. Most of the time though this is moot unless in an emergency, since the Hammerhead will usually try to stay outside the range of these weapons anyway.
In 4th and 5th Editions, the Hammerhead competed with Broadside Battlesuits for heavy-support slots. For roughly the same cost, a trio of Broadsides could put three twin-linked railgun shots on target compared to the single-shot Hammerhead. However, the Hammerhead could move further before shooting, and could fire an S6 AP4 Large Blast template to vaporize light infantry, while Broadsides could only, at best, bump off three soldiers with a lucky round of shooting. For the 6th Edition Codex, Games Workshop decided to "resolve" the conflict (read: give people a reason to buy both Broadsides and Hammerheads) by changing the Broadside's main weapon to the "heavy rail rifle" -- nerfed to S8, but with the option of getting Skyfire, making them a threat to flyers in a way that a lone Hammerhead never could be.
Comparison to the Leman Russ
Design wise, it's interesting to note that while the Russ looks like (as the russes 1d4 chan articles says) "A drunken three-way between an old British Mark-V, a M3 Lee, and a T-34." the Hammerhead looks more like the love child conceived after a night of red wine between a M1 Abrams and the U.S. Navy rail gun program and the child was then spoiled rotten by grandmam Attack helicopter.
Much like a cold war tank the Hammerhead is low to the ground with good (if the model is any indication) gun depression, armed with a mostly anti tank gun with some anti-infantry shell options, while being blisteringly fast. Further, it's role is more like a western US tank as compared to a Soviet one as western tanks were built to take on lots, and lots of Soviet tanks at once, as mentioned Hammerheads are almost always drastically out numbered by Leman Russes, Which is very ironic for the often communist accused Tau.
Tactically they fill the same niche as a modern tank, an element supporting mechanized infantry advancing very, very quickly taking out armor while using it's machine guns on infantry. The Ion cannon version fills a role more like a Bradly's chain gun in taking out light infantry, and (if a modern military ever faced power armor) heavy infantry. The Lemen russ on the other hand is a world war two tank and it shows, it's much slower and has a much broader silhouette but it is better armed and armored.
The Russ's can generally do only one thing well, be it destroy tanks, infantry or fortifications, meaning the Imperial guard needs, in order to cover it's base's three tanks, a Normal Russ, a Vanquisher, and a Demolisher to be effective. But the Hammerhead is good for only one task, namely destroying other tanks, sure it's got sub munitions but it's main role is destroying other tanks so that it can support and work with infantry, or battle suit to accomplish the objective.