Terran Unit Roles (vs. Protoss)
Terran versus Protoss is mainly a Barracks, Bionic (Bio), centered match-up due to Marines and Marauders being extremely effective against all Gateway units, in equal value. And in the mid-game Terran has Stimpack and Medivacs to compound this natural advantage.
Some pressure and defensive builds to employ Banshees and Siege Tanks for early harass and defense but still depend on Marines as the mineral backbone of the army; and in the later game the mobility of Bionic units can be used to bring down a Protoss's production and economy more effectively than a frontal push with Siege Tanks can manage.
In the late game, it is accepted that an unharassed Protoss will have the advantage because of the Warpgate mechanic allowing for a quick army re-max on Zealots and Archons is too strong for Terran Bionic or Mech to handle without a large amount of Ghosts that will destroy a Terran's overall army value making them sightly worst at the late game.
 Early Game
Besides some early game Warpgate or Immortal pushes for breaking Bunker defense Protoss does not have any distinct early openings without greedy fast Nexus (3) builds. But, this weakens Protoss against Medivac-harass and forces them to be aggressive on low-tech Gateway armies giving them the same problem as the aggressive openings.
So, Protoss standard opening involve either Double-Force or Colossus openings to upgrade Gateway armies or the splash damage needed to deal with Bionic armies. This opening has the famous 10 minute mass-Marine and Siege Tank timing weakness; while Protoss only has one Colossus and no Extended Thermal Lance to deal with Marines easily. But, Terran needs to All-In on one base to do this.
With Stimpack and Medivacs complete Terran can now freely push out on the map, avoiding direct army battles. Protoss will need to keep its army together as Marines and Marauders can snipe any number of high value targets like Nexus, Colossus, Tech structures, etc. being very costly in buying time and economy for Protoss on two bases.
This harassment will buy time for Terran to take an extra mining base, Ghost/Viking tech and Engineering Bay upgrades. Taking Terran solidily into the late-game with a slight tech or economic advantage.
Good all-round. Primary defense unit in the early game. Used in combination with Marauders to make up the meat of a typical bio Terran army. Early game Marines can out micro Zealots, but will be out micro'd by Stalkers
Extremely effective vs Stalkers, and also good vs other Gateway units when used in combination with Marines. Makes up the rest of a typical Terran bio army. Concussive shells makes retreating from Marauders extremely difficult.
Necessary in sustaining a Bio army and used to drop. Their healing is essential to keep the front line alive vs Psionic Storms, Zealots, and Colossus. TvP is a very positional match-up, and the mobility that stimming provides is important to get cost effective engagements vs the Protoss ball, and healing this damage is crucial. Additionally, dropping is extremely important to control a Protoss economy and tech; picking off valuable structures such as Forges can be very effective.
Essential for dealing with Colossus. When the Protoss player has more than 2 Colossi, their damage output is too large for bio to deal with when protected by a Gateway force. Vikings can abuse their extreme range to pick off unwary Colossus. Large numbers of Vikings are important because Colossus can kill a Terran army extremely quickly if not dealt with.
Essential in late game TvP because of their EMP ability which removes up to 100 shields from every Protoss unit hit, and removes 100 energy. Necessary to deal with High Templar-based play. Snipe is often used to neutralise High Templars.
- Siege Tank: Have a strong advantage against Stalkers and Colossus, but force Terran into a defensive opening or a 2-3 Siege Tank push timing on one base. The sheer amount of Vespene Gas severely delays Terran Engineering Bay upgrades.
- So it is unlikely Terran will make more than 2-3 Siege Tanks for a standard Two Tank Push opening.
- In the mid game, Siege Tanks are simply useless against Zealot bum-rushes and give Protoss the engagement advantage because Bionic needs to be micro-ed away from Zealots and Archons.
- Banshee: Have a strong advantage for harassing Protoss with Cloaking Field upgrade and are faster than Protoss's fly detection, the Observers. Banshees kill Sentries and Probes extremely quickly and both have about the same value but targeting anything else takes too long and harass timing is limited by Banshees energy.
- Banshees kill Stalkers in one on one fights, but take too long to build to be a long term solution for harass.
- Raven: Point Defense Drone is the only use Ravens have for some timing attacks to combine with a Siege Tank push or After Banshee Harass openings. PDD's stop Stalker and Immortal attacks and placing the PDD near a Nexus (2) is the while point of this timing attack, known as the Polt Timing Attack.
- Thor: Occasionally seen for stomping Sentry Force Fields with a big Bionic army to attempt to break the Protoss's natural before Medivacs are available. Thors suffer from sever weakness to Feedback spell and Immortal and Stalker attacks, making them very hard to get in any decent number.
- Reaper: Strictly in the late game only, when Protoss has 4+ bases. Reapers can rip through Probe lines and escape quickly; with extremely high damage output against Zealots, Sentries and High Templar. And, have arguably more mobility than Blink Stalkers; but this depends on the layout of the map.
- But suffer from the same problem as Banshees, having a long build time and difficult to mass again.