The terrans are a very versatile and flexible faction. They possess a varied set of armaments, while the majority of their units can fight enemies both on the ground and in the air. On the other hand, though, their basic units have very little hit points and therefore need to move in large groups to accomplish anything. The terrans have units capable of cloaking, so they are able to attack hostiles without the risk of taking casualties, and they are also able to use nuclear weapons.
Each unit uses a certain amount of Supplies. For example, a Marine or an SCV use only 1 Supply point, but Battlecruisers use as many as 6. Building a Command Center and a Supply Depots increases the force limit. It's a good idea to create an economic infrastructure beforehand to avoid any slowdowns while raising an army.
The special ability of many terran structures is the ability to move to another place (Lift). Every such building has its own set of engines and can therefore be transported to a new spot, for example, when you want to evacuate a base during a hostile attack or to move it closer to the frontline. You can also lift the misplaced constructions or completely rearrange the entire base if need be. This ability proves useful when you don't have enough minerals to erect a new Command Center but you still want to shorten the mineral transportation time - you just have to move the base closer to the raw material deposits.
This faction can invent many technologies that enhance basic unit types. Scientific research takes place in special buildings and their extensions. It's a good idea to erect more than one structure or a given type to speed up the process. However, it also inflicts higher costs. Also, the base will be larger and you'll have to increase its defensiveness accordingly.
Terran armies are divided in an important way. Infantry belongs to biological units. In their base version, they can't be healed in any way (Medic is added in the Brood War expansion). The vast majority of terran units are vehicles that can be repaired by SCVs. They use small amounts of resources but it's still cheaper than recruiting a new unit. Terran buildings, in turn, begin to burn when severely damaged. An unrepaired building will eventually be destroyed in this way.
Strong defense is the strength of the terrans. An SCV can build Bunkers in which footmen can hide and fire on the enemy. Two enemies full of Marines can stop most enemies. Turrets detect hidden enemies and have powerful antiaircraft rockets. Playing this faction, you should build many Supply Depots and Bunkers around your base. In this way, hostiles won't break into your base and you'll protect your workers and production buildings. You can also rely on producing Vultures which are very fast and can use the hit-and-run tactic. Their special ability is the possibility to plant hidden landmines. This allows you to secure the area around the base without endangering your army.
While attacking, you have many useful units at your disposal. Initially, large groups of Marines are enough to carry out nearly any operation. However, with time, you'll notice that they are too delicate compared to other units. Siege Tanks should be the basic units of any assault team. They work well both for attack and defense. In the Siege Mode, it deals great area-of-effect damage and has a quite long range. A big group of tanks can destroy infantry and buildings in a blink of an eye. To support them, send in Marines or Goliaths (strong antiaircraft rockets) that will deal with an enemy air force. You can also decide for a slow trench assault. It involves building a line of bunkers increasingly close the enemy position and filling them with infantry and repairing them whenever necessary. In later stages of the campaign, every assault squad should consist of at least one Science Vessel. Not only does it have its "magic", like boosting a unit with a special protective field, but also possesses the Detector passive ability. Thanks to it, you can detect invisible or underground units (mines or the zerg).
Powerful air units of the terrans make conquest easier. A Battlecruiser is a mobile air fortress. Having developed a special technology, it gains access to the Yamato Cannon which can destroy any enemy turret with a single shot while staying out of their range. In this way, you can get rid of threats without engaging them with land units. The Wraith fighter and the Ghost infantry can also Cloak. Few enemy units can detect invisible enemies so you can wreak havoc without taking unnecessary casualties. What's more, the Ghost infantry has a very useful ability - Lockdown. The target stops moving or attacking for a short time. Use this to stop a very strong hostile unit (i.e. a Battlecruiser) and summon your forces to destroy it.
The last very important element of the terran arsenal are nuclear missiles. They require a special extension for your Command Center and a large amount of raw materials and Supply points. They also require an observer, which is a Ghost, that will designate the target. Large buildings take two rockets before they are completely destroyed but the great area-of-effect makes up for this. Nukes are perfect for clearing an area from groups of small units or to weaken grand enemies and fortified enemy bases.