StarCraft The Terrans
The Siege Tank is the Terrans' most devastating ground unit. It can also convert itself into a siege cannon - more than doubling its attack strength. However, these strengths come at the cost of a couple of major vulnerabilities. Siege Tanks have no air defense capabilities, so you will need to protect them with Marines or Goliaths. Also, Siege Tanks in siege mode have no short-range attack capability. This means that they are extremely vulnerable to ground units when in this mode. Keep some Marines close by to prevent your Siege Tanks from being swarmed.
The basic Terran air unit is the Wraith - a very versatile unit. You can use Wraiths to provide air cover for your ground forces, defend against enemy units, or as an effective strike force. The Wraith's cloaking ability is a strong asset that you should fully exploit. Use cloaked Wraiths to harass approaching forces, scout enemy territory, or disrupt enemy supply lines. Just beware that enemy detector units such as missile turrets will be able to see your cloaked Wraiths.
The Battlecruiser is the most expensive Terran unit, and arguably the most powerful when armed with the Yamato gun. This long range weapon can quickly reduce an enemy base to rubble. Target outlying missile turrets with your Battlecruisers - they'll be able to take them out at long range with their Yamato guns, paving the way for your other air units. Since Battlecruisers are so expensive, it is generally a good idea to provide them with some escort to draw enemy fire. A fleet of Battlecruisers with Wraiths for escort is a formidable force indeed.
The Science Vessel is unique in that it has no traditional weaponry. It does possess several special abilities, though, which make it a nice addition to the Terran fleet. The first of these is Irradiation which envelopes an area in deadly radiation. Use this to wipe out clusters of units quite quickly. Avoid unleashing it on solitary units, though, as the energy cost of this weapon is pretty high and shouldn't be wasted. Next we have the EMP Shockwave. Use the Shockwave against units with shielding to remove their shields and leave them vulnerable to attack from your other units. Obviously this ability will not help you much against the Zerg, but it is quite an asset against the Protoss. Finally, the Science Vessel carries the Defensive Matrix. It places a protective shield around the unit of your choice. Use it to protect your Dropships as they make their way into hostile territory or to protect the vanguard of your assaults.