Sentry Turret
Sentry Turrets appear as miniature tripod robots that appear in the Portal series. They are incapable of independent movement, but can open each side of their chassis (their 'arms') horizontally in order to reveal two gatling guns. The central part of the chassis holds their red 'eye', which emits a targeting laser. Almost all of the space within the turret is devoted to storing ammo.
When Turrets are unaware of the player's presence, they will maintain a standby state and will remain motionless with their guns retracted. Moving into their angle of sight however, will cause Turrets to deploy their guns with a voice line before opening fire. They will continue to fire until the player is dead or moves out of sight. If the latter occurs, Turrets will briefly search for the player (as indicated by the movement of the targeting laser), after which they will revert to a standby state.
Turrets can be avoided, disabled or destroyed in a number of ways. The most common method is knocking them over, causing them to flail and fire wildly before deactivating. This can be done by picking them up and dropping them, or using any object to push them. Additionally, the Thermal Discouragement Beam or Emancipation Grid can be used to completely destroy them. It should be noted that once a Turret has been knocked over and deactivated, restoring it to an upright position will reactivate it and the Turret will function normally once again.
Rocket Turret
The Rocket Turret is a unique enemy in Portal. It appears as a personality core outfitted with a rocket launcher. Upon activation, it will slowly lock onto Chell and emit a sound before launching a rocket. It plays a vital role in the battle with GLaDOS, as rockets need to be sent through Portals in order to hit her, causing personality cores to detach. Rockets can also shatter glass.
Defective Turret
Defective Turrets, also called "Crap Turrets" by Wheatley, are incomplete, ammo-less versions of Sentry Turrets. Unlike normal Sentry Turrets, Defective Turrets have a more humorous and sarcastic dialogue, and speak in a masculine, less robotic voice.
They are harmless to Chell, many times making comments such as "If anyone asks, I killed you". Their voice lines seem to imply they are blind, but they are still aware of Chell's presence. They are first encountered in Chapter 5: The Escape.
Frankenturret
Frankenturrets are bizarre Sentry Turret-Weighted Storage Cube hybrids created by Wheatley in order to complete test chambers when he had no test subjects. They are first introduced in Chapter 8: The Itch.
Frankenturrets do not speak, and instead emit incomprehensible chirping sounds. They act as the Chapter's replacement for Storage Cubes, their only difference being that Frankenturrets will hop around aimlessly while not placed on a button. When picked up by Chell, they retract their legs inside their cubes. If they fall over on their sides or upside down, they will wave their legs trying to get up.
Prima Donna Turret
The Prima Donna Turret (literally First Woman in Italian, is the title given to the Lead Singer, the Diva, in an opera) is a bigger, wider turret that is featured at the end of Portal 2 in the turret choir, after GLaDOS lets Chell to the surface.
Apart from the ending, a Prima Donna Turret can also be found in Chapter 3, Test Chamber 11. There is a Prima Donna turret in the elevator at the end prior to the player entering the room. It can also be found below a Ratman den in Chapter 3, Test Chamber 16, where it is observing a turret quartet.
Animal King Turret
The Animal King is a unique turret that is only referred to in an Aperture Science slideshow entired "Animal King Takeover" in Chapter 1: The Courtesy Call. The Turret is considerably larger than its Sentry Turret brethren, and appears to operate in exactly the same way. It can be distinguished from ordinary turrets by its immense size, crown and leopard skin-detailed chassis. In the context of the slideshow, the Animal King Turret is used as an example of an entity or organisation capable of taking control of Earth which "...refuses to, or is incapable of listening to reason".
The Animal Turret appears only once at the end of the game, when Chell is travelling to the surface after defeating Wheatley.
Hover Turret
The Hover Turret is an unused Turret found within the game files of Portal 2. It emits a laser, similar to the other types of Turrets, that is coloured light-blue and able to burn the player similar to the burn of the Thermal Discouragement Beam, however unlike the Thermal Discouragement Beam it's laser does not provide a barrier for the player's movement, nor does it interact with game mechanics. The Hover Turret can be spawned within Portal 2 via the cheat command ent_create npc_hover_turret.
