Security Cameras, also known as Spy Cameras, are collectibles in Sleeping Dogs.
Dating Not Ping will show the locations of the cameras on the minimap.
You hack the cameras after you have defeated the thugs guarding them and waiting for a drug dealer to come along. If you hack the camera you can later go to your apartment/house boat and access the camera and bust a drug deal, arresting the drug dealer.
There are also other cameras to hack that aren't part of the drug busts, but the hacked cameras are not usable.