A Security camera is a device that watches for prisoners, such as Nick. The first camera is the level Welcome to the Movement. It is guarded by a Meat Puppet. There are several more in the level Fears Don't Lie in the area before you confront Wei Lu. There is also one camera in From the Ether and Beyond.
Camera's are mounted on a wall and turn left and right. They watch out for anything wrong. If Nick is spotted by one an Alarm will sound and Meat Puppets are summoned.
There are two main way to destroy cameras.
Many players simply open fire on it. This often ends with the camera spotting them, and the alarm sounding. A camera can be destroyed with the silenced pistol without set off alarms.
Gas canisters and explosives barrels can quickly destroy a camera.