A dongle is a small hardware device (but sometimes software-based), often plugged into a USB port, that acts as a "key" allowing one to use a particular software. ProTools, for instance, ships with a dongle, which must be plugged in, in order to use their software. Dongles are an attempt to dissuade the use of pirated software, but the ever diligent hacker community has always found ways around these challenges and dongles provide little resistance to the underground pirate community. Dongles keep the honest people honest, but thats about it.