There are two types of Turrets in Brink; portable, deployable Auto Turrets and fixed Sentry Turrets defending each team's spawn.
Auto Turrets
Engineer's turret.
Turrets are part of the Engineer abilities set, and can be constructed and placed on the battlefield. They autonomously scan the immediate area in an arc, and upon seeing an enemy combatant, fire at will.
Turrets can be upgraded to be more powerful and more difficult to destroy; the earliest available form being about equivalent to the firepower of a Submachine Gun, with the second and third incarnations resembling an assault rifle then heavy machine gun, respectively.
While turrets can be deadly, they really need to be supported by the Engineer constructing them due to their somewhat limited scanning range and speed. Light Body Types will be able to outrun or avoid their line of sight if they time it correctly. Operatives will be able to hack turrets and reprogram them or disable them using EMP grenades at higher Ranks.
Sentry Turrets
Sentry Turret.
In order to prevent one team from directly attacking the other's spawn point and preventing them from even attempting to complete their objective, there are fixed Sentry Turrets. These are both invulnerable and incredibly lethal, safely securing spawn areas for players to get their bearings before moving out.
Players wandering too close to enemy spawn areas will receive a verbal warning informing them that they are leaving the area. If they do not heed this warning then the chances of them surviving confrontation with the Sentry Turrets are slim.