When you read strategy guides, most of them concentrate on specific matchups or even build orders. Usually, guides don't cover the "basics", based on a widespread but obviously false belief that most players know them. However, the basics, or fundamentals, as I prefer to name them, have many levels. One of them is the purely technical level - how well do you micro/control your units or how many APMs you have. Another is however the understanding of game mechanics - here, many players seem to play without understanding key concepts, getting good results only due to sheer technical advantage / game experience.
When you watch a Brood War replay, you tend to know which player is generally winning at the moment. "Generally winning" is an abstraction of a key concept of the game - advantage. Here, I'd like to focus on two aspects of advantage in StarCraft - tactical advantage and strategical advantage. I'll try to discuss how to negate one using the other, how to build up each of them and how to transform them into each other.
First of all, however, I have to explain what tactical and strategical advantage means. Tactical advantage is the advantage determied by unit numbers, choice, position, upgrades and static defenses. Strategical advantage is the advantage determied by mining capacity, upgrade capabilities and the amount/placement of unit construction facilities, as well as the general tech level. This seems obvious and many will now ask "but how does it relate to game advantage like we see it"? So, here's an explanation of how your winning game of StarCraft should proceed.
At the start of the game, you usually want to build up strategical advantage. Your goal during a balanced game is to maintain strategical advantage over your opponent, while not having a significant tactical disadvantage. Then, you will want to turn your strategical advantage into tactical advantage, then proceed to finish off your opponent.
All of this looks pretty abstract, so let's look at some real examples.
Let's say a player does a 5 pool rush vs. a terran opponent who goes a balanced, 10-12 rax build. The moment the zerg player stops building drones and makes his SP, he's putting himself at a big strategical disadvantage. The moment he gets his lings hatched, he gains a big tactical advantage. He now has to use those units to build up a strategical advantage or he will lose. So, he rushes to the terran base, but the terran find the lings with a scouting SCV. Since they are far away, the terran manages to build a marine and put it in his mineral line before the zerglings arrive there. The tactical situtation is now a draw, but the terran holds a large strategical advantage = game winning situation.
The general rule of StarCraft (or 2 rules, to be more precise) regarding strategy and tactics are:
1. Building up tactical advantage means sacrificing strategical advantage and vice-versa.
2. You need to build up a decisive tactical advantage to break through your opponent's defenses, but the game is won by building a decisive strategical advantage.
So, what are the means of building up each type of advantage? We'll start with tactical advantage first.
The first and foremost means of building tactical advantage is the behavior known as "consuming". This means using up all your production facilities to produce units, as well as devoting all resources to those means. This causes your unit count to rise, which will obviously build up your tactical advantage.
Another means of building tactical advantage is unit placement. For example, in PvP, with an equal strategical situation, the protoss that's containing his opponent has a big tactical advantage, even with similar unit count, resulting typically in a game-winning situation.
Combined with unit placement is the idea of drops. Drops build up tactical advantage by delivering units in places where they are hardest to defend against. This is generally just an aspect of unit placement, in a way.
How do you build strategical advantage, then? First of all, as you probably guessed, the opposite of consuming, the technique known as "powering", builds up your strategical advantage. Powering means devoting all your resources to increasing your production and resource-mining capabilities - making workers, expansions, additional unit-construction facilities is all part of powering.
Teching is yet another way of gaining strategical advantage. It allows you for a wider unit choice, therefore expanding your production capabilities.
So, now that we know the theory, how to we forge it into practice? Well, the idea is first of all to know when we're at a strategical or tactical disadvantage, second of all know the balance between the tactical and strategical situation in-game, and last but not least know how to use that knowledge to secure a victory.
The first part involves a concept known as scouting and map control. If we know what our opponent is doing and know where his expansions/production facilities/troops lie and what he can produce, we know how we stand with our in-game situation. The more we're kept in the dark regarding our opponent's situation, the more we have to adapt to a wider possible range of tactical/strategical advantage/disadvantage, meaning that we cannot plan our actions well enough.
The second one is something I'd call "game sense". This allows us to judge whether our production is better than our opponent's or what will be the result of a potential battle of our amassed troops. Knowing that is crucial to planning a successful strategy - making a brilliant plan won't help if we're convinced that our contain will hold and then the opponent manages to break out with almost no losses.
The last part requires us to use the "StarCraft rule of balance", which is: each action that builds up a strategical advantage also puts you at a tactical disadvantage and vice-versa. Actually, on an equal level of technical play, most people lose because they either don't know or neglect this rule. Let me show it on an example.
A zerg player manages to contain a protoss, who has just set up an expansion but has no observers. Both players are at 1 expansion, giving the zerg a serious tactical advantage and the protoss player a quite big strategical advantage. Now, the zerg player can do two things. He can either consume and create troops, building up his tactical advantage or he can use the containment to create expansions, putting himself at a strategical advantage. Let's see how this affects game play.
If the zerg choses to enhance his tactical advantage, he will sacrifice even more strategical advantage. At some point, the protoss player (having better production capabilities), will, apart from keeping his strategical advantage, start nullifying the tactical advantage the zerg has. At some point, the protoss player will gain tactical advantage and break out. Then, having both tactical and strategical advantage, he will proceed to win the game.
If the zerg uses the containment to build up a strategical advantage, the protoss player's prior advantage will nullify the tactical advantage even faster. But this time, the zerg player has been building up a strategical advantage, so at the moment the protoss breaks out, the zerg player might have enough strategical advantage to switch to consume mode and nullify the tactical advantage the protoss has gained by breaking out.
Most people lose their games by needlessly furthering their tactical advantage or their strategical advantage, instead of balancing them where there is a chance. Now, there's nothing wrong in building advantage, but remember the "StarCraft rule of balance" - there's always a cost. This is sometimes seen in a situation of a protoss player who gains advantage vs. a zerg with only a natural expansion. then mass expands, then loses the game because the zerg gets hive tech and starts dropping zerglings everywhere. I just saw a game where a terran player rushed (successfully) a zerg, forcing him to cancel his expansion hatchery (giving him a big strategical advantage), then, instead of using that to build tactical advantage (simply putting it, make enough troops with a balanced build up), he tried to fast expand to even more increase his strategical advantage. He put himself at such a big tactical disadvantage that the zerg player simply rushed him with zerglings. This is also a typical situation of a PvZ game with the protoss doing a purely tech build.
Of course, things aren't as simple as that. You might ask - well, how am I to break through my opponent's defenses if I don't build up tactical advantage. Well, that's one part I kind of lied to you about earlier. There are two types of tactical advantage, actually - global tactical advantage and local tactical advantage. Global tactical advantage is one you see when you look at the entire map. Local tactical advantage is something you see when you look at a single game screen. A typical example is a PvT game. If a protoss has a flank set up on the middle of the map, with shuttles ready to drop zealots inside a terran's push, and the terran has all his troops in his natural, the global tactical advantage belongs to the protoss. Moreover, the protoss has a large local tactical advantage on the middle. However, when we move to the terran natural, the terran has a slight tactical advantage there, meaning he will be able to defend any attack made by the protoss player.
Now, it's the time to introduce the rule of the thumb on the advantage balance: never build up global tactical advantage that will not give you enough local tactical advantage at your opponent's defense point to break their defenses. The same goes the other way - if you build up strategical advantage, always try to sacrifice global tactical advantage instead of local tactical advantage at your defense point. This rule is very often used by a 'diversion' tactic - if you do a drop at the same time as doing your expansions, you're sacrificing global tactical advantage (your general troops), but you're not really putting yourself at a big local tactical disadvantage since your opponent needs to pull back his troops to defend the drop, which gives you the time to mount a local defense (build up local tactical advantage) at your expansions. This is also the key rule for all the "camped-up zerg ZvP winners" who use very significant local tactical advantage (sunken/lurker defense) to offset the big global tactical disadvantage they suffer by powering/teching rapidly.
So, where's the part I lied to you about? Well, it's the part about drops. I said drops are basically only a means of unit placement. Well, drops are means of building up big local tactical advantage as well. If I use 2 arbiters to recall my troops over a huge lurker contain, I'm building up local tactical advantage in a place of strategical importance and therefore changing the balance in global tactical advantage very drastically.
I hope I've shed some light on the "internals" of StarCraft strategy. What I suggest is, after reading this, try looking at some replays of gosu players and see how they maintain their balance between tactical and strategical advantage - it can be an illuminating experience if you look at it this way...