Will detect most fires much more rapidly than a heat detector.
Two most common types of smoke detectors are ionization and photoelectric smoke detectors.
Ionization smoke detectors. Have small amounts of radioactive material in the sensing chamber that ionizes the air and renders it conductive with current flowing through the chamber. When smoke particles enter the ionization chamber, a sharp drop in current occurs which, in turn, sounds the alarm. (Majority residential.)
Photoelectric smoke detector. A light source and a photosensitive sensor are arranged so that the rays from the light source do not normally fall on the sensor. When smoke enters the light path, light is scattered onto the photosensitive device, causing the detector to respond.
Additional information about Smoke alarms
FPT 102 - Fire Prevention and Inspection // Instructor: Kevin M. Kolb Slide #10