I hear people talking a lot about having map control (mostly tzenes). What specifically does a good player consider map control? Is it simply having decent visibility of most of the map so that you know whenever your opponent is moving a force around the map? Or is it having expansions/fighting units around the map? How will I know when I have map control?
So map control is a very ephemeral concept, but at its core is the idea that you can control space (area) on the map and prevent things like: expansions, harassment, scouting.
Since map control is largely concerned with covering area, having map control usually means having units which are very fast and thus can respond quickly. Additionally, it often times means having good vision on most of the map. Its important to note, that Vision helps you maintain map control, but in and of itself it doesn't give you control. Control is the ability to respond quickly.
Consider the following scenario: You're a Terran player who went MMM. You have a large force and its sitting in the center of the map.
Now compare that player to a Terran player who went 1:1:1 and has a lot of Vikings. All of a sudden he can be everywhere very quickly. Reaper Harass? Vikings land in the front and back and you've got him. Overseer scouts? shut down. You control the map and what happens on it.
Map control is very powerful not only because you can scout but also because you can expand. If I'm starting up a new expansion away from my main, I have the choice of putting my army there, or in my main, or splitting it up. The first two options lead to suffering from harassment, the last one suffers from having poor response to a big push. However, if you have units which can quickly move around the map and shut down harassment, all of a sudden you are free to expand and out Macro your opponent. This is very much a dominate strategy in Broodwars.
Some units which help with map control are:
but this is by no means an exhaustive list.
A number of people have said that vision connotes map control, or helps a slow force respond quickly.
While vision is nice, it is ultimately a scouting technique. If you have sufficient scouting it is still possible for an opponent to launch successful harassment if you don't have the units to respond.
Since map control is based around the Ability to Respond, having vision is largely beneficial, it reducing your response time. Additionally, being able to scout your opponents base also reduces response time. Neither of these things, in and of themselves, gives you control. The only determining factor is: can you respond?
In answer to this question I largely focused on what units gave map control instead of how to scout as scouting is largely a different question. This is not to understate its importance though.
tzenes and Carl posted on vision and ability to respond, which are very important. I think "map control" covers that and a bit more.
The way I think of map control is, how much of the map do you "own" and how much does your opponent "own." What do I mean by ownership? In my mind, it's like a game of Risk. In Risk, it's obvious who owns a territory because they're the only one with troops there. In Starcraft it's less clear-cut but I think maps can be divided in a similar way.
For each part of the map, you can estimate who has more control (or potential control) over it. Who has better intel/vision of it, who actually has troops there, who could have troops there quickly, who has expanded there or near there, and so on. It's hard to analyze because you can "own" part of the map even if you have no vision or troops there, as long as you know what's there, and have the ability to respond should you need to. But on the other hand, the less you have vision and the less you solidify your ownership with troops, the easier it is for the enemy to gain control.
Another way to think of it is that your map control is your sphere of influence, your area of command, the extent of the map where what you say goes. This starts in your base, and extends around your expansion and armies, but it can also cover stretches of the map where you have no direct presence, as long as you can exert more control there than your opponent at a given moment in time. For example, if your opponent has turtled and you have him bottled up, and have good intel on his air units, you effectively control the whole map, even though you only have units in a few places.
There are more lessons from Risk we can apply in Starcraft (Australia is how many of us learned about choke points!)... but for now, I find the mental picture of map control as claiming Risk territories helpful.
Having map control simply means how much you know. You can see what your opponent is doing at all times.
You don't need to be able to defend all portions of the map simultaneously, but your recon gives you enough information to tell you where your opponent is moving in and you can relocate your forces to defend as necessary.
Some people say that having more expansions than your opponent is map control. I disagree, there's no benefit to having an expansion which you can't defend in time because you never saw your opponent heading towards it. Having more expansions is simply a benefit that comes from controlling the map. In addition, you can choose where your battles occur because they happen on your terms. You don't need to "control" choke points, rather you need to be able to move to them in time and choose your battle to occur at that location.
Early game, the easiest ways to gain map control are:
Later on you can also gain map control with: