[cure]

In this article I'm going to outline the standard formations; their strengths and vulnerabilities, as well as some useful tactical maneuvers and important marching orders.

Primary Formations

Concave Arc
The ideal formation you would want while attacking someone. The arc maximises the number of troops able to engage.

Spread
The ideal formation to face banelings and infestors, however the worst formation to be in against zerglings, as you expose the surface area that is available for surrounds.

Ball
The ball formation is what your army is usually in. This formation is ideal when focusing down a target directly infront of the ball, where all your units are able to fire, or when attempting to kill flying units.

File
This file formation usually occurs when you move your army out to attack and some units are faster then the rest, or when you funnel through a choke. This formation can easily be changed to the arc formation.

Tactical Maneuvers

The Standard Flank
Most formations and marching orders you will face will be vulnerable to a flank attack. The flank attack will constrict your opponent's movement and disperse his firepower's focus.

The Multi-Prong
The Multi-Prong comes in many different ways. The basic idea of the Multi-Prong is to attack your opponent on different fronts at once. For example, sending a small force to attack your enemy's expo while attacking his base.
-Other Examples-
Terran can drop the main base of his opponent while engaging the main army.
Zergs can nydus or overlord drop while attacking.
Protoss can send in DTs while attacking or use a warp prism.

The Retard Magnet
The Retard Magnet also comes in many ways. The basic idea of the Retard Magnet is to draw your enemy out into the open and catch him in an unfavourable position. For example, sending a single unit to your opponent's main to bait him into moving his army there,
then proceed to snipe an expo/structure.
-Other Examples-
Terran - Sending an empty dropship into the main of the enemy to make him bring units over.
Zerg - Faking overlord drops / Nydus worms. Using an overlord to bait the protoss units out of position, then proceed to surround with zerglings.
Protoss - Send a real voidray along with 2-3 hallucinated voidrays to make your opponent bring alot more units then he needs.

The Full Surround
The Full Surround is used when you need to constrict your opponent's movement completely. For example, using speed zerglings to surround hellions so they're unable to cause anymore damage. Also the full surround maximises the surface area of attack for the zerglings,
which will result in more damage being dealt and less units running in circles attempting to get a hit on the enemy unit.

How To Prevent Them?

Preventing the flank and the full surround
Use the terrain to your advantage, you could "hug" impassable terrain to guard the flank. Eg, stay close to cliffs so the enemy is unable to get a full surround.
However, smart opponents will wait till there is no terrain that you can abuse to flank you. In this scenario, if you don't have map control, its wise to have a unit in the front, back, left and right of your army to watch for flanks, so that you can react earlier. (Fig #)

Preventing The Multi-Prong
Preventing the Multi-Prong is not an easy task. However, it can be made easy with map control. If you have vision of the maps, you're able to see when he's attempting to multi-prong you, and you can send units ahead of time to defend against the impending attack.

Preventing The Retard Magnet
To prevent the Retard Magnet, simply send no more then you need to kill the bait. It's relatively simple to spot a magnet. When you see a single unit at your expo, just send a single unit to handle it.

How do i keep my army formation if i have to move from point A to point B?
Whenever you click on an area, your army will stack into a ball when it reaches the location. The nearer you click, the faster your army goes into the ball formation, the further you click, the slower your army will move into the ball formation.

Last words: Remember to use terrain advantage as much as possible, remember to position your army before attacking / defending, and always watch for those drops ! Remember these key points as they are crucial even in high GM.