This will help you best understand what Terran units are used for based on what tiers they are in. A general overview of each unit within the Terran arsenal for Starcraft 2
Tier 1
SCV – SCV’s are your mining and gas harvesting units, along with your building constructors. You should make 2 SCV’s per mineral patch (don’t make 3 per patch because it is not worth it mathematically, just expand if you need more minerals). To select idle SCV’s, press F1. If your enemy comes into your base and attacks your SCV Mineral Line, make sure to immediately select all of your SCV’s and attack the enemy units. A lot of players freeze up and don’t do anything when their SVC’s get attacked, or try to run their SCV’s away, but it is better to attack with them.
Mules – You should always upgrade your first Command Center to an Orbital Command Center so you can get Mules and Scanner Sweep. Whenever your base has enough energy, you should call down mules directly onto your largest mineral patch (to keep your minerals running out evenly). If your enemy attacks your base, you can even call down mules onto your base’s wall so that your enemy gets distracted and attacks your Mules and gives your wall more time before you can reinforce it with combat units.
Marines – Great all around unit and the most common Terran combat unit, they attack both air and ground. They go well with Marauders, Ghosts and Medivacs. Make sure to research Stim Packs if you go mass Marines.
Marauders – Marauders wrecks all ground units, and are especially good at focus firing Colossus. They cannot attack air, so if your opponent switches to air, stop making them and switch to Marines or other anti-air units such as Vikings or Thors. Research Concussive Shells against Protoss right away, though Concussive Shells are not as important against Zerg as Zerg units already move very slow when not on Creep. Very good in combo with Marines, Ghosts, and Medivacs.
Reapers – Reaper rushing your opponent with one or two reapers is a standard opening play on maps that are small and where your opponent’s base is close to yours. Otherwise, they are not a good unit to mass.
Hellions – Great against Zerg units like Zerglings, Roaches, and . Especially good in combination with mass Thors as they share the same upgrades and act as a good distraction/meat shield. They are very fast so they are good to use to harass your opponents expansions. Hellions do not work well with Marine/Marauder builds as they require a different building to produce as well as a different upgrades.
Tier 2
Ghosts – 2 to 4 Ghosts are a must for and Marines/Marauders build to add EMP support, especially against Protoss as EMP’s do a lot of damage to Protoss Shields and fry the mana of High Templars. They are great at harassing your opponents base if they don’t have detection. Do not get Nukes with Ghosts, Nukes just aren’t effective enough. Even if you can stay cloaked long enough to Nuke your opponent’s mineral line, you won’t destroy their buildings and they will move their miners before it falls. Nukes cost too much to build and the resource are better spend on more combat units.
Banshees – Banshees are air units but they can only attack ground units. Banshees with the Cloak upgrade are good to get against early Zerg armies of Zerglings and Roaches since those units cannot attack air. If your opponent has a mainly ground army, Cloaked Banshees can be good to get as long as your opponent hasn’t made Protoss Observers or Zerg Overseers. They work well in combo with Vikings as Vikings will shred enemy air units and protect the Banshees. Do not get Banshees if your opponent masses Hydralisks, or Stalkers are those units will shred your Banshees.
Vikings – Great as anti-air units against Zerg Brood Lords, and Protoss Motherships. Also great for taking down Protoss Colossus. They are very weak against Zerg Hydralisk and Protoss Stalkers. Good to pair with Vikings, and good as Anti Air support for Marauders.
Medivacs – Medivacs are overrated. They don’t heal fast enough during combat to make much of a difference, and you can to build a Starport to get them which are resources down the drain. It is better to just build more Marines and Marauders and do more damage, than build a Starport and Medivacs.
Ravens – With their Seeker Missile ability and Cloaked Unit Detection, they are a great addition to any army facing mass ground units such as Zealots/Stalkers/Sentries or Roaches/Hydralisks. Don’t tech to them right away though, build a nice force of Tier 1 units such as Marines and Marauders before adding them to support your forces.
Siege Tanks – Siege Tanks are not as powerful as they were in Starcraft 1, but they are still a decent unit to get when facing mass ground forces. When using them in combat, put them into Siege Mode before the fight starts, run Marines into your enemy, then draw them back into the range of the Tanks.
Tier 3
Thors – Thors are most effective in groups, and with Hellion support. With their 250mm Cannon Stub ability, they are great for taking down groups of Ultralisks, and Colossus. Also very strong against Air units such as Mutalisks, Vikings, and Banshees. If your Zerg opponent goes Infestors, make sure to focus down the Infestors or your Thors will fall under their Neural Parasite Mind Control ability.
Battle Cruisers – These units are very hard to get, and are not very cost effective. They are a very overrated unit. You should not be playing with builds that use Battle Cruisers, the game will be long over before you are able to get a significant number of them.
The video is an explanation of all units within the Terran arsenal for Starcraft 2.