A smoke detector is simply a device that detects smoke. Smoke is usually an indication of fire. This is not always the case and when there is no actual fire they can be responsible for a number of false alarms.
This device can be a local only type where they are installed to notify anyone that is in the proximity of the smoke detector. Detectors can also be connected to a control panel which can be used to sound an alarm in an entire building.
These types of systems can also notify authorities if the system is monitored by an outside agency.