Terran is the most diverse of the races and contains strong counters and healing units. Terran also posses the strongest early game unit, firebats. Firebat coupled with sentries and/or roaches provide a very strong opening. An early bio push, firebat/marauder/medivac with roach/sentry can be game ending. After pylon terran players can focus on using their strong counters or forcing a counter by massing tanks or banshees. Dingles and Medivacs are useful healers. Commandos stun opponents. The major disadvantage of terran is its weak very late game air units.
Marine[]
Basic infantry unit. A technical pre-requisite for Marauder, Firebat, Ghost, and Hellion.
Because of the upgrade mechanics in NW, Marine DPS is second to Carriers (supply for supply). Every attack upgrade increases their attack by +2, meaning their attack doubles at the third upgrade. That's roughly the cost of one Battle Cruiser, a relatively useless unit.
Reaper[]
Anti-light infantry unit. Detector. Strong early-on, but loses efficacy quickly. Good against early light units and sentries in one lane. Not effective in two-lane play though.
Firebat[]
Front-line tanking anti-light infantry unit. Likely the most powerful unit early-on, but becomes useless later. Early fireboat pushes supported by roaches, sentries, and/or queens can be game ending.
Ghost[]
Specialized anti-light anti-biological infantry unit. Uses Snipe.
Can Cloak when researched. Cloak consumes no energy and is permanent.
Commandos have the stun ability. Commando/thor is the a very powerful combination when engaging mass scourge strategies.
In one lane ghost/commandos dominate the game. It is not uncommon for players to mass ghost/commando especially when playing zerg opponents. This is hard countered by AOE attackers, storm assassins and colossus. But the extra income given from ghosts allows for a very strong transition to Dingle/Mothercore/Carrier which is the most powerful unit composition in the game especially against mass colossus.
Hellion // Hellbat[]
Anti-light vehicle, micros itself. Technical prerequisite for Siege Tank, Thor and Medivac.
☀Should never build more than one.
Siege Tank[]
Technical prerequisite for Warhound. Mass siege tank can provide a very powerful mid-game push.
Mech timing attacks based on upgrading before 900 pylon can be game ending, especially when coupled with immortals or ultras.
Middling with tanks is very powerful and common.
Warhound[]
Anti-armor mechanical unit. Strong against immortals and Mech units.
Warhounds hard counter Thors, Tanks, Immortals, Stalkers, and Sentries.
☀Personally, Warhounds are my favorite unit in the game.They are definitely the most underrated
Thor[]
Heavy mechanical unit. No major weaknesses. Very strong against scourge. Thor is the best all around ground unit in game.
Remember Thors get countered by Warhounds.
Science Vessel (Dingle)[]
Flyer which heals friendly mechanical units. Great when you have air control
Medivac[]
Technical prerequisite for Banshee, Viking, Battlecruiser and Science Vessel (Dingle). Heals friendly biological units. Can be upgraded to heal two friendly units at once.
Should never build more than one.
Banshee[]
Cloaked Air to Ground attacker with high DPS. Can be very strong situationally.
Good Ol Desperate Banshee (GOD B): desperate times call for desperate measures. If you're getting hard pushed by a heavy ground opponent who lacks air detection (mother cores for example), start middling banshees. It'll force your opponent to awkwardly get out not only AA, but also air detection. This can buy you time to get back in the game. After forcing out air units, you can do really well by tech switching to Thors.
Viking[]
Air to air attacker. Anti-air. Strong against single use targets such as carriers. But consider building thorn against cluster targets such as Mutas or Scourge.
Battlecruiser[]
Never build them. Repeat after me: "NEVER BUILD THEM".