SC2 player, want to get into BW

BW to SC2 conversion guide: Terran

Rax units
SC2 marines are goddamn monsters. They do a massive amount of DPS, and can stutterstep to move while shooting. However, because medivacs are worse at healing (and you have fewer than you would medics), you have to be careful about over-stimming them. In early GSLs, zerg players would attack with mutas, and Terrans would stim THEIR ENTIRE ARMY to deal with it, depleting all their medivac energy, which is a pretty good reason to not have your whole army on one hotkey. Even if you're bad, it's hard to accidentally stim 50 marines at once in BW, and you can get away with it a bit more, too. In TvZ, you want to be dropping marines as much as possible on one side of the map while attacking on the other. Bunker rushing is actually standard TvZ, but Zergs have gotten good at defending that. Damn good. A marine-scv all-in used to be standard during the dark times of SC2 (GSL3), but pro zergs learned how to deal with it and the ladder doesn't use it as much because they're copying the pros. (And because of the popularity of the Spanishiwa opening on the ladder for Zergs)

Marauders are beefy, but are really good at killing armored units and buildings. Marauder drops can thus be very effective at sniping tech buildings, especially against protoss. In TvT, Marauders can break lines if the tanks don't have marine support, and juggling marine/marauder/tank numbers is an important part of TvT. Marauders are also how bronze-level players defend against banelings, but it's not the best way to do it.

Reapers are a unit that doesn't have an SC2 equivalent. It's a really fast unit that kills light units pretty well, and is ludicrously powerful against buildings for some reason. They can also jump up and down cliffs, so it's hard to kill one of the Terran is good. The combination of cheap, fast, and able to avoid ramps makes a reaper great as a scout, but it's rare to get lots of them at a pro level (though I see mass reaper as a counter to Spanishiwa. I've yet to lose to it, though, and I don't think it's good as a build)

Ghosts are way better in SC2. They're easier to get, and can EMP protoss. Their snipe ability deals 45 damage to a bio unit, can be recast almost instantly and is a little underused. Ghost rushes are sometimes viable, but they're mainly lategame TvP or TvZ.

Factory Units
After spider mines were all executed as traitors, someone strapped a firebat to a vulture and called it a hellion. Hellions don't have mines, but they're great at killing light units like marines and workers. The blue flame upgrade increases that bonus, making them fucking ludicrous at killing workers. 4 blue flame hellions in a medivac can and often do kill 20+ workers. Lately, players are discovering you can micro them against marines a little, too, and they're pretty brutal. Hellion drop into banshee is a standard aggressive opening in TvT, but hellions are great against Zerg and Protoss, too. However, as the game goes one, they find themselves less useful (usually) without spider mines, and tend to fall to the wayside.

Tanks are tanks are tanks. They're not as powerful as BW tanks, but they're smarter. SC2 Terran is generally less inclined to kill itself than BW Terran can be.

The Thor is a giant-ass slow-ass goliath. They do splash damage to air, and are thus often used against mass muta. Mutalisks can magic box a thor to avoid the splash (Magic Box in SC2 does the opposite of magic box in BW). If you go mass Thor in TvZ, Brood Lords will kill you. If the Zerg forgets about Brood Lords (like I tend to), Thors kill everything else with ludicrous cost-efficiency. In TvP, thors aren't used that often, especially now that Templar can feedback them for massive damage. It used to be that the only way for a protoss to counter mass thor was to go mass immortal, making Thors the only unit Blizzard has nerfed (making them feedbackable) for the stated reason of being incredibly fucking boring otherwise. I don't approve of them. Luckily, you don't see mass thor in pro games. Thank christ.

Starport Units
Vikings are flying units that can only hit air. They can transform into ground units that can only hit ground, making them the only unit in either game that can drop itself. Tank/Viking was a common TvT strategy a little while ago, but it turns out drops are really good against that. Nowadays, Vikings are gotten more re-actively against Brood Lords or Colossi. In TvT, a player going viking/tank will transition into viking/battlecruiser, to which the other Terran counters by going mass Valkryie (with Thors? It doesn't happen often). Valkyries can BARELY kite DCs, leading to some cool-looking BC/Viking micro that I haven't seen a lot of. Players who open banshee against Zerg (somewhat rare) usually get a viking to kll overlords with as a followup.

Banshees are flying units that hit ground hard and can cloak. They're flying, ranged DTs. As I mentioned, Hellion into Banshee is a common SC2 opening, and can sometimes be a closing as well. It's pretty common to see both players in a TvT go banshee. Microing a banshee against marines is a little hard. Microing marines against a banshee is a little hard. Microing marines against a banshee in your base while microing a banshee against marines in his base is really really hard, making banshee v banshee one of the more mechanically difficult openings.

Ravens are the science vessel of SC2, but have completely different spells. Point Defense drone is a gimpy dark swarm, and auto-turret drops a turret that does decent damage and lasts like ten years if not killed). The upgradable seeker missile is fucking amazing against tank lines, and otherwise useless against people who can micro. Ravens aren't that common, but I saw a game at Dreamhack where both players went mass Raven/Viking for some ungodly reason. It was bizarre and a little awesome.

SC2 battlecruisers are fucking ninjas. They are way way faster than the BW counterpart. Late game TvT can often turn into BC/Viking, but it's rarely seen in other matchups.