Here is the breakdown of the complecated aspects of the terran army. Its true, the terran army is by far the most difficult to command affectively. But, it can also be the most deadly threat when wielded by an experienced player. If there were one single reason that make terrans so difficult to command is the fact that there is no single unit that you can build to wipe out the enemy (battle cruisers are an excpeption to this, but they require insane amounts of resources that are just not available on a regular map ) Instead, you must utalize several groups of units at once to acheive results. Not to mention the fact that terran units don't heal by themselves, you must heal them manually. This makes micro management skills a must when using the terrans.

When you first enter the fray, you have only a command center and four scv's to work with. Make a change by starting a new scv and gathering minerals with your other scvs. Build up to about 7 or 8 scvs and once again you're game will differ from here.

1. You can start a barracks and a refinery and go straight for medics and marines for a rush
2. You can build a barracks and start some bunkers for defense and prepare for a bigger scale battle.

Note* If you are fighting against a computer, you can try this buffer trick: If your base has a slim choke point, build supply depos next to each other and block off the choke point with them. Then immediatly build a few bunkers behind the depos. The enemy will start to flood in and your marines will fire at them with relative safety. The computer units are dumb and they try to run by the depos to get to the nearest threat: those bunkers you built. But since they are blocked off, they will just run around in front of the depos until they are all killed by your bunkers. Be careful when using siege tanks this way, as the splash damage can destroy your depos, ending the buffer affect quickly.

* Firebats work well early in the game when you be defending against wave upon wave of zealots and zerglings-that is if you can get in an acadamy fast enough.

Expanding to new mineral locations

The terrans are kinda pricey, so an early expansion might be the ticket. Just make sure you have adequate protection at both bases. When you find your new mineral location, check around for some choke points. If there is one, build some bunkers first for some protection. Bringing a brigade of marines and a few siege tanks makes your expansion much safer from an attack. Have a defensive network in place before you even start gathering minerals there. Building an extra barracks and an extra factory at the expansion helps produce extra units when the enemy wears your front line down too much. Always keep a pair of scvs hotkeyed so you can select them quickly for repairs when you are under attack by force.

Note: Since the terran buildings have the ability to fly, you can build all the extra structures you need for the expansion inside the safety of your starting base. Then when you are ready to expand to a new location, you can build a few quick bunkers for defense while your buildings fly in. Be sure to have adequate detection.

Building and upgrading

Once you have secured an adequate amount of resources, you will want to start thinking about building your war machine. If you plan on a large land invasion, you will need plenty of factories and barracks. Also you will need a pair of engineering bays and armories to upgrade your units to the maximum level. Dont skimp on the supply depos-they become very annoying later in the game when you want to build a large fleet quickly. If you want an air invasion, do the same, only with a lot of starports instead of factories. Air units are doubly expensive, so you'll really want to upgrade their weapons and defenses so they will last as long as possible in combat.

*Once you start your upgrades, hurry and get that science facility in because all level 2 and 3 upgrades arent availble until you have it.

*Leave a space between factories and stargates so you will have room enough to build thier addons

Defending

Defending is what the terrans do best with their mammoth siege tanks and bunkers full of upgraded marines, coupled with spider mines and compact missle turrets. Build lines of bunkers across choke points and back them up with siege tanks and missle turrets. For an added line of protection and an early warning system, dot some spider mines around the outside of your bunkers' line of sight-this also spots for the long range siege tanks. To protect against an air attack, build a wall of missle turrets. They can pummel attacking air units before they go down. When fighting the zerg, keep a dozen wraiths or so around to repel a guardian attack, they work particularly well because of their unique cloaking ability.

* Use the science vessels irradiate ability on the hoards of enemy units flooding into your base. The irradiation cloud does splash damage to biological ground and air units.

* Valkalries make excellent anti-air units-just be sure that they have ground cover. 

Attacking

This field is where it gets difficult. If you do not attack with the right combination of units all in unison, you will definitely lose the entire group to enemy defenders. With the exeption of large fleets of battlecruisers, no one terran unit can affectively engage the enemy. When using ground units, the siege tanks push is unsurpassed. Build a dozen siege tanks and matching sets of marines, goliaths, and wraiths and move to just outside the enemy line. Siege your tanks and just sit tight. Your tanks will punish the enemy and they will probably send some units to foil your plans. This is where all those extra units come in. Use your marines, goliaths and wraiths to keep they enemy off of your tanks. Use a group of scvs with medic support to keep all your mechanical units Dent-free. IF you lose any one leg of troops, you might as well back off because you will probably lose everything without all those units working in unison. With air units, numbers are your only hope. Build huge fleets of battle cruisers, backed with some wraiths if your money is low. 12 Wraiths will only die a quick (and exspensive) death at the hands of only a couple photon cannons. If you are going all air, upgraded battle cruisers are a must. When you have your air fleet, send them at the enemy base enmasse from the weakest angle, so your enemy will have a harder time repeling the attack. If your fleet starts to take too much damage, return to base for repairs.

Note: Battlecruisers can use their yamato cannons to kill missle turrets and spore colonies from outside their range.