Terran vs Terran

Massive Marines
  Against large amounts of Marines and/or Firebats with StimPack  build a lot of Bunkers in a close formation throughout your town. Your Bunkers should be built  in tight clusters so that each Bunker is able to attack anything attacking  the next nearest Bunker. If you place too much space between your Bunkers, the enemy can easily  overpower them one by one.

   When you are attacked, have all your SCVs repair your Bunkers. Repeatedly right click on the Bunkers to  keep your SCVs repairing them. If you don't repeatedly right click, the SCVs will stop repairing once  they've restored the Bunker to full Hit Points  and might run away when attacked. This Bunker repairing method should allow you to survive any early attacks  as long as your SCVs mining are protected. Place a few Bunkers around your SCVs mining.

   Try to have your Marines Stim Packed inside the Bunkers. Unload them, StimPack, then get back inside  the Bunker.

   Once you've defended against a Marine rush, you have several options:  try to rush as quickly as possible to Vultures, research Spider Mines and lay  down formations of Mines outside your town. You can also choose to bypass the Vultures and head as  quickly as possible to Siege Tanks, or do a mixture of both. Vultures are only 25 Minerals more  apiece than a Marine. Combined with Spider Mines, and the ability to repair Vultures, enemy Marines/Firebats should  be toast. You will need to build 2 or more Factories to quickly build enough Vultures to take the  Marine threat because they will most likely have 3-4 Barracks building Marines.

Beware: Siege Tanks Ahead!
  You should take care to make sure your enemy does not beat you to Siege Tanks. If you don't continue your  push up the tech tree, you may find you are quickly attacked by lines of Siege Tanks outside your   town, which is often very difficult to stop. Build a few Siege Tanks with Siege Mode for defense, then  find out what the enemy is building for. If they are trying for a Siege Tank battle, make sure  you have more and use them more effectively. Use Ghosts to Lockdown enemy Tanks, Defensive Matrix  your own Tanks to make them last longer against enemy Tanks. Use Wraiths to attack them if the enemy does not   go for air targets or doesn't protect them properly with Marines.

Control the High Ground
  If there is high ground around your town, you must gain control of it, or prevent the enemy from gaining  control of it. Something to watch out for is people who will defend their town, and then race as quickly as   possible to Siege Tanks and Dropships. If you don't watch out, before you know it they will have unloaded Siege   Tanks above your town to fire on it. If you don't have Dropships yourself, and you cannot reach those Tanks,   your only choice will be to lift-off your buildings and move out of range. This is really something you  should watch out for. If you have Wraiths available and the enemy doesn't defend their Siege Tanks  you can use Wraiths to counter the attack. If you have no Wraiths available, but have a Dropship, place  Marines in the Dropship and unload them directly on top of the Siege Tanks where they won't be able  to return fire. If the enemy supports their Siege Tanks with Marines, you might not be able to fight them.  Use Siege Tanks on low ground combined with ComSat to reveal the Tanks on High Ground. If you have  more Tanks than the enemy, it might be possible to fire up the hill and destroy the Tanks.

Cloaked Wraiths
  You shouldn't have too much trouble with cloaked Wraiths if you make use of the ComSat Station and hot keying.  If you get attacked by Cloaked Wraiths, move Marines into the area, StimPack them and use ComSat to  reveal the cloaked Wraiths. That should   quickly get rid of of Wraith threat (if you have enough Marines). You can also use Missile Turrets to spot for   cloaked Wraiths but you may find that using ComSat and Marines is easier and faster. Missile Towers are usually  most effective if they are spread out across your town to prevent Ghosts from sneaking in, and to chase off  Dropships. If you decide to race to cloaked Wraiths, you may find the best use for your Wraiths is to attack mining SCVs and Siege Tanks.

   To defend against cloaked Wraiths, it's very important to have several Bunkers protecting  your SCVs mining. If you do not, the enemy can quickly destroy all your SCVs. You should also try to build  one or two Missile Turrets around that area to detect the Wraiths. If the Bunkers are attacked, repair them  with your SCVs. If the SCVs are attacked, run them around the Bunkers making the enemy chase them. Pop your Marines  out of the Bunker, StimPack, then get back inside. 

Marine Drop
  Protect your resources against the Dropship/Marine drop. Quite often the Terran enemy will load  8 Marines into a Dropship, then drop them off in the middle of your mining SCVs. Combined with StimPack,  if you don't have any nearby defenses this will quickly kill all your SCVs and even your Command Center,  placing you back into the Stone Age and possibly resulting in a
mobility kill.  Place a few Missile Turrets around your resources (they're a bargain at only 100 Minerals) and a few Bunkers there to take on any invasions. 

Siege Tank Spotting
  When fighting other Siege Tanks, it's VERY important to have range on them, meaning you can see and fire on  them before they fire on you. This means using the ComSat Station or other units such as Dropships  to reveal the enemy for your Siege Tanks to fire. Quite often Terran vs Terran games become Tank wars.  It is up to you to have MORE Tanks than the enemy and use them more effectively by taking advantage  of high ground, and further sight/firing through the use of spotters.

Vultures
  Remember, Vultures are very good for use against Marines, and Firebats. Spider Mines can be used to  fight land troops of all kinds, although large groups of ranged units on Attack-Move may be able to  destroy Mines as they activate. If the enemy does this to you, use ComSat to reveal the Mines, and use  Siege Tanks or Vultures to clear the Mines. (Vultures will not set the enemy Mines off). A large group  of Marines on Attack-Move can also be used as Mine-sweepers, but tight clusters of Mines will still pose  a problem. 

   Vultures with the Ion Thrusters upgrade are excellent worker assassins. With their fast speed, they can be used  to move past or around defenses and stop the enemy's mining operations. Bunkers will chew up Vultures, though, so  this won't work if your opponent's SCVs are protected. Vultures can also easily and  quickly scout the map and watch resource spots to prevent the enemy from taking them over.

Fighting Lots of Battlecruisers
  The enemy may decide to defend their town and, as quickly as they can, build large numbers of Battlecruisers.  If they are sloppy in their cloak detection, you can used cloaked Wraiths against them, but skilled players  will use a Science Vessel to reveal your cloaked Wraiths or use a ComSat Station to reveal them. Another  defense against this, (and probably the best) is to use cloaked Ghosts to Lockdown the Battlecruisers,  then use Stim Packed Marines, Wraiths, and/or Goliaths to finish them off. EMP can also be used to knock  out the Yamato cannons on enemy Cruisers. You can also use a larger number of Battlecruisers yourself.