Cloaking Devices are devices (du-hur) that use special fancy physics to render things invisible. They are usually mounted on starships, but are also commonly utilized by rapists, widely successful bank robbers, and galactic supersoldiers.
Although totally frickin' awesome, cloaking devices have their limitations. You need a serious power source to run a decent starship cloak, and you can't run your cloak and your microwave at the same time. A ship under cloak cannot generally use shields or weapons. Cloaking devices also require highly trained personnel to keep them working or to repair them when damaged and such skilled personnel are usually total shitwads. Small, personal cloaking devices have far fewer limitations and as such people utilizing them can just get away with fucking murder.
Various types of cloaks have been invented by races all throughout the galaxy. The Romulans were the first race in the Alpha/Beta Quadrant powers to be seen (lol) using them. They found them so widely successful that today there isn't a vehicle in the Romulan Star Empire not outfitted with a cloak. This makes catching the ice cream man very difficult for Romulan children. Many adolescents on Romulus have been driven mad - following the music for days in the vain hope of chipwich or a rocket pop.