Containing is a tactic that involves placing units or buildings in positions that makes it difficult for opposing units to leave an area. The opposing forces are required to initiate battle under unfavorable conditions (usually from within a main or natural base). A contain can range from simply setting up a concave at a choke point to creating an entrenched position by building static defenses and artificial choke points.
The main purpose of a contain is to gain map control. Once achieved, the player executing the contain is free to expand without fear of being attacked at their base. Since the opponent is effectively trapped inside their own base, the opponent's expansion will be very delayed, thus giving the containing player a large economic edge.
Breaking a Contain
Using brute force to break a contain, while possible, is usually not advisable. The way in which a contain is set up will almost always create favorable battle conditions (such as Choke Points and Unit Positioning) for the containing player in a straight-up engagement. If possible, it is best to use alternate means to leave your base, thus circumventing the contain. Possible escape methods include:
After successfully leaving your base, you can either use your position to flank the containing units, or move in for a counterattack on the enemy base. A counterattack forces the enemy to abandon their entrenched position, allowing you to retake map control and expand. If the enemy does not scout you leaving your own base and counterattacking theirs, it is likely that your attack will result in a base trade situation.
Common Contains by Race
Technically, a contain can be achieved with any unit. However, these are some of the more commonly used contain methods.
(Note: Terran may wish to add Engineering bays or supply depot walls to their contains to further restrict enemy movement. Terran contains also commonly include Missile Turrets for detection and anti-air)