The Siege Tank has two modes of attack: Tank Mode and Siege Mode. Siege Mode must be first researched at the Machine Shop. Both modes are only capable of damaging land units. A Siege Tank in Tank Mode deals 30 explosive damage, and one that is in Siege Mode deals 70 explosive damage plus splash. These can further be upgraded at the Armory.
While in Siege Mode, the Siege Tank is incapable of movement and gains a range of 12, but also has a minimum range of 2. This means melee units can attack Tanks without being dealt damage by the Tank. The Siege Tank is able to freely transform between the two modes with a short transformation sequence. Opponents will attempt to attack Tank Mode Siege Tanks due to the lower damage dealt. A few Siege Tanks will quickly add up exponentially in the amount of damage dealt because of the significant splash radius.
A Siege Tank in Siege Mode has the greatest attack range in StarCraft. It is even greater than its sight range, so it benefits from units such as Science Vessels or lifted off buildings, which give it more sight.
Researched at the Machine Shop.
Tanks are universally effective in all competitive match-ups. Siege mode significantly increases their effectiveness, so it is also researched.
Siege Tanks are particularly potent against the Zerg because their land units are generally weak (low health hit points). As such, the Zerg player will often augment their ground attacks against Zerg, whether it be using Dark Swarm to reduce the damage of the Siege Tank in Seige Mode or drop attacks directly right on top of the Tank or utilize air attacks, including Spawn Broodling.
Tanks are particularly effective in clearing out Sunken Colonies in standard M&M Terran due to their longer range. They also prove very effective against Lurkers with Detection. Siege Tanks don't have to be in Seige Mode to be effective against Burrowed Lurkers. Surprisingly they outrange the Lurkers even in Tank Mode.
If the Zerg chooses mass Hydralisks over Mutalisks, Siege Tanks will easily dominate the battle as they are very effective in their Seige Mode against Hydralisks. In a standard Mech build, the Terran usually supplements its mass Tanks with Goliaths. The best Zerg response to this is Defilers with Dark Swarm and Plague, but even these are not as effective against Mech as against the Bio infantry army. This is because Tank splash damage will still devastate mass Hydralisks and Zerglings. Additionally, Goliaths are effective against Mutalisks, who are not protected by the Dark Swarm. Plague is a greater threat, weakening Tanks and Goliaths to be finished off by Mutalisk or ground unit attacks.
Note that burrowed units like Lurkers under Dark Swarm take no damage from Siege Tanks, thus Mech will need Science Vessel support against Lurkers and Defilers.
The Siege Tank is a staple of standard TvT. By the mid game, many TvT match-ups will involve over a dozen Factories on each side. The battle field and chokepoint will often feature lines of Siege Tanks in defensive positions, with Goliath, Wraith, Vulture, or even Battlecruiser support.
Dropships are regularly employed to avoid the Tank's powerful ground attack and exploit its lack of mobility and an anti-air attack by dropping Tanks in Tank Mode and other ground units directly on top of the sieged enemy Tanks.
During the early game, the Terran player can use two Siege Tanks in Seige Mode and two Wraiths to kill an enemy Siege Tank in one volley, with Wraiths providing vision for the Tanks and dealing the finishing blow at the same time.
Two Siege Tanks in Seige Mode with a +2 attack upgrade will kill an enemy Tank with only one volley, regardless of its armor.
By the late game, where large Siege Tank formations will reach an impasse, the Terran players may opt for mass Wraiths, as they can easily clear an area of Siege Tanks in Seige Mode before they have an opportunity to escape. As such, Siege Tanks are frequently also supported by Missile Turrets.
The Siege Tank is also a staple of standard TvP match-ups focusing on ground forces. Its 70 damage with splash decimates Dragoons and other ground units that comprise a standard Protoss army in this match-up. In PvT, Siege Tanks are usually accompanied by Vultures, which soak up damage and protect against Zealots and Dragoons with both direct attack and their Spider Mines.
Siege Tanks can also be used to disrupt mining. Because mining Probes have few Hit Points and are usually clustered together, a few siege mode blasts can devastate a Protoss mineral line. Siege Tanks' excellent range make them ideal for assaulting Protoss mineral lines from convenient drop points or across ridges or cliffs. Probes will easily fall to a Siege Mode shot, even when upgraded to +3 armor/shields, making well-orchestrated Tank drops very devastating against Protoss.
Some Terran users claim a hidden exploit exists where Seige Tanks in Seige Mode can minimize the number of shots fired against approaching forces. This calls for the Terran player to repeatedly use Shift+Attack to direct the group of Tanks against a series of units (e.g. Dragoons) in the approaching ground force. This claim has not been fully verified.