This first thread will be about the terrans and their backbone units marines. There are always different ways of executing things depending on situation. But hopefully it can provide some help in deciding how to use your units.
Marines got a huge damage per second countering their price and a great movement speed with stim. The only thing that really outshines them are zerglings, but marines are ranged and that is huge, specially in larger numbers. Now this guide will not tell you if you should retreat or attack more how you should try to micro in a fight.
The early game is more how you counter certian units vs other units, later on it's more about position (placing "tankier" units at the front) since both players will have several units.
--- early ---
Now we start with something simple, in the early game you will problary have marines and marauders, now you will need stim to get superior movement speed (but don't overuse stim) since marines will deplete Hp pretty quick.
A lot of players have screamed of imbaness over terrans "A-move". It's not really what we will do, but it's sort of true. You Attack-move the ground in front of your units and imediatly move back with your army and then Attack-move again. This is called kiting. Versus most units in the early game you will use defencive kiting. The only few examples are:
Stalkers: if you have succesefully kited all the zealots the protoss players loses a lot of their dps and "tank-power" and protoss player now tries to run you should always try to pick of those stalkers in order to actually do damage to a protoss player. What you do instead of attackingmoving and then retreating you do the opposite you attackmove and then move forward and then attackmoving again. This will hopefully land a few extra concussive shells and kill of more stalkers then it would otherwise do because of stalkers great mobility.
Zerglings: these cheap and huge dps speeddemons can easily kill a stimmed marine group if you just try to kite them. What you do is that you try to move as close toghether as you can and if you got time clump yourself at a wall which will increase your marine survivability tremendous. So instead of trying to kite zerglings decrease the directions in which they can attack.
Banelings: Now there is no quick guide to dealing with banelings, here it's more about speed and fast thinking. But what you basicly want to do is spread out your units but only the front units since if you spread out every unit zerglings will quickly surround them and kill. If you got armored units like marauders you should try to position them in the front.
*You create a "mini-wall" with units that will hopefully hold the baneling wall while you keep the other group of your army toghether and follow the standard vs ordinary zerglings.
--- mid ---
Now you might have gotten siegetanks or medivacs (remember this only covers the most basic terran armies consisting of marines) and there are different ways of approaching the way you play now. Either you focus on spreading your army into 2 groups, one "normal" army and the other army as "specialised" army. (Now I don't mean spellcasters this time)
The normal group could be consisting of marines / medivacs and the other group could be marauders or non-sieged tanks. What you want to do here is that your marines mainly just attacks and the marauders fire at that huge and armored colossus that would deal a tremendous amount of damage if not taken care of.
*Basicly A-move and all units that deal +damage to armored should be focus fireing armored targets.
A pretty common strategy is the so called "MMM-ball" now it's pretty similar to the early game kiting. You kite your enemy with your only ranged army and marauders and marines does this very well. But one common problem I see is that they keep their medivacs in the same unit group, doesn't seem bad does it? Well the problem is that medivacs could be seen as a unit with "0" cooldown meaning they don't benefit from moving and then "attacking", they will lose healing time. So splitting your marine+marauder army from your medivacs will make the medivacs constantly healing while your kite with the marine and marauders.