In the early days of multiplayer, Terran was a one note race. You build marines, you build marauders, and you build medivacs. Then, you hit t (hotkey for stim) and you a-moved for the win. Now, there is a new style of play taking form in all the matchups, especially Terran vs. Terran. It revolves around mech play.
The basic premise of Mech is to build a critical mass of factory units and rely on superior positioning and macro to win. This is in stark contrast of the micro-intensive m/m/m where one has to be afraid of any and all aoe units. In TvT the use of siege tanks allows for a player to deny drops and secure positions while using Hellions to harass and gain the upper hand and using Thors to soak up damage and break enemy siege lines.
Remember that if you play mech, your damage dealers are your siege tanks, they have the highest dps unsieged and their splash damage while sieged provides you with a great way to decimate any clumped up balls of units. Because of this, it is imperative that you keep your tanks safe and only advance when there is no threat of any kill-squad sniping of a stray tank. A normal mech macro set-up is 3 factories off of one base, two with tech labs, one with a reactor. When one hits 2 base, one can add two more factories. Use this as a rule of thumb, each base can support two factories while simultaneously keeping hellion production up.
Thors and hellions are there to keep the siege tanks safe. Thors provide the AoE splash damage air attacks that should ward off any banshees and vikings allowing the mech player to maintain air control. Remember that even though Thors are good at this kind of anti-air, producing vikings allows a player to gain vision, allowing the mech player to gain the vision advantage which is vital for their siege tanks.
Blue Flame Hellion Mech:
This build revolves around getting six blue flame hellions to put a player on the defensive and cause massive economic damage. This build is great on big maps like Tal'darim Alter are when is far away from eachother by air, as this build is very weak against a banshee opening.
10 - Supply Depot
12 - Barracks
13 - Refinery
15 - Orbital
16 - Marine
Upon Orbital completion - Second gas
Upon 100 gas - Factory
Maintain constant marine production until 4
Scout after third marine with SCV
Build Bunker if needed
Upon 100 gas again - 2nd Factory
Get a tech lab on the first factory and begin blue flame research immediately upon completion
Begin hellion production on the first factory after completion of tech lab
Get a reactor on the second factory after finished, if down properly, there will just be enough money for all this
Stockpile until six hellions, then move out.