Technically first showing up in Portal 2 (there was a malfunctioning turret in the first game, and this turret is debated to be a defective turret), a defective turret is a turret that is defected in any way to where it's incapable of functioning properly. There are two types of defective turrets: the upright turret that's assembled correctly but is missing its outer shell, and the turret with its main body attached sideways on its legs. Both are missing bullets and do not have working "eyes."
The second version of the defective turret is not seen much, and you don't really get to interact with it in the game (maybe once or twice), but the first version is quite the opposite. They play a huge role in the game when trying to shut down the turret assembly line, and have many, many humorous lines. Another interesting thing to note about the second version is that it appears to be rather old and battered, judging by the scorch mark.
The defective turrets claim to be blind numerous times, but they somehow still notice Chell. The defective turrets seem to have a very different voice than normal turrets, and it is much more masculine, as opposed to the quiet, female turrets. Also, it is very important that you don't get the defective turrets and the Oracle turret mixed up, for the Oracle Turret is "different" and does not fall under the defective category.
***WARNING: Even though it has spoiler filters, the website linked for the dialogue of the turrets DOES contain spoilers. please continue at your own risk.***
In Portal 3, there are several more types of defective turrets. These include a turret that has bullets, but seems to have terrible eyesight, and often shoots for no apparent reason. Another type introduced in Portal 3 is a turret with no legs. Some have bullets, making the turret actually more dangerous than the normal one, as they cannot be knocked over, but most have none.