Okay so before the game starts, there is a strategy. A plan. A mode of operation if you will. That mode of operation is allowed by the units you select...which means... the units you select must allow you to act in the mode that you want to.
Seems simple enough right?
Okay, so then before you ever SEE the battlefield and its terrain or how that can affect you; and before you touch pen to paper for a list, you are required to figure out how you want to BEST the opponent.
After looking through the rules there are a few ways to do it, in the strategic sense, and those strategies inform the list building. Most armies are some varying degree of reflection on all these thoughts, but probably fall towards one style more than the others:
1. Morale. A melee oriented force will attempt to win through crushing the enemies morale in a lightning Blitzkrieg sort of way. They do not necessarily intend to actually slay every last person they see through the volume of dice, but instead the QUALITTY of the dice they are throwing. Their intention is to grapple them into submission and/or use dirty tricks to break their actual will. You see this attempted by things like Deathwing armies, who might have smaller squads whose goal is to simply survive long enough to win a close combat by 2 or 3 and sweep the enemy away. You can see it in some Dark Eldar armies who attempt a blitzkrieg. They are not actually doing, numerically, all that many ACTUAL wounds when you look at at in totality. Armies like this employ tank shocks and Psyker powers to force morale checks and even wargear in some places. Anything to basically minimize the amount of deaths necessary in order to gain the result. Paladin lists kind of represent this strategem at work where they may only kill a couple units when its all said and done, and some HQ's butthey are qualitative kills.
2. Casualties: Morale is a component certainly, but some armies appear to be more about creating overwhelming numbers of wounds such that the enemy simply can't save enough. They dont tend to be the tougher (higher armor or toughness) armies that try this, as they often will invest heavily into either high attack killers like Yrmgrals and Khornate berzerkers. They may come in the form of 5 AssaultCannon wielding Speeder units with 5 heavy bolters. Some Purifier builds attempt this, causing copious wounds in order to simply dissolve the threats into separate puddles so weak they are simply combat inneffective. Armies that try to win through pure casualties are most penalized by a lack of targets and you will often see people simpy use reserves to counter this to some extent, but such armies can perhaps take advantage of speed (such as in the case of the speeders), allowing them to weather the inevitable hail from their enemies as they close in to contest or just from their own lack of numbers. Those usiung the strategy will be the most likely to be called one trick ponies as they will feature in many cases, repetitions of what the owner perceives to be the "A List" models in the codex. You'll hear people talk about "redundancy" a lot if they build a list to this strategem.
3. Position: Physics is clear on the matter: you simply can't have two things in the same place at the same time. Blocking the enemy and shaping the battlefield using the terrain at hand, the immovable objects at hand [b]and[/b] well placed units like Scouts forward, infiltrators on flanks etc... It can be kind of an intrguing way to win. From so many angles, the enemy is unable to focus its force on any one element and from so many angles, you are unfocused but in position to thrust where the vulnerability lies. Such armies are kind of rare, but they are effective in that they can put pressure on so many different units and like a good basketball teams, seek out the mismatches and try to exploit them. Fearless blobs and other such units can all make this kind of warfare work by entrenching enemies in a certain spot and blocking others. Necron Scarab farms are a good example of how this can be effective, and Tyranid Tervigons and Gaunt pods can be another way that you try to do this. You need not even win the close combats. Just stand your ground long enough in the right places and disallow the enemy to get anywhere ELSE. Also this kind of strategy gives you a lot more ROOM for falling back, so failing your morale checks doesn't derail the strategy as much as with other ones, where failed morale checks can be a much more serious matter. Chaos Daemons kind of used this strategy in their old incarnation.
4. Timing. I have made pretty extensive use of the strategy that [i]one cannot be killed if one is not there to receive the sword blow[/i]. As a Tau player I have loved this kind of sneaky warfare where you kind of ABLATE the enemies strengths by disallowing them from actually causing any damage or "doing their thing" very well. As the rounds go by, they start to recognize how little they have killed out of your (typically high) model count. Through the expedient of deep strikes, outflanks and late arriving transports, you can push the enemy back and take your prize. Timing your arrivals to minimize enemy efficacy is the key to playing this way. Battles like this end with a lot of fast and furious action. Feints are used early to draw attention just enough to force action without offering real gains in return. This is a little harder to do because frankly, your innate wish to roll a lot of dice and watch your enemy pick up models gets in the way of you WANTING to do this. But it is effective for some armies, Tau in particular, and can be the epitome of cat and mouse. An important point in any game where you try this is to recall that games aren't infinite. This is why the strategy works. The enemy can't get more rolls than the game allows. Going second is advisable here though its not required.