There are two main types of smoke detectors connected to a fire alarm system; Ionization smoke detectors and Photoelectric smoke detectors.
Ionization smoke detectors are best used for sensing fast flaming fires that give off very little smoke. Photoelectric smoke detectors are best used where slow smoldering fires with thicker smoke might occur.
Ideally a combination of both types of smoke detectors should be installed within a fire alarm system.
There are other differing smoke detectors called Multi Criteria smoke detectors. These smoke detectors can have a Photoelectric detector and Heat detector within the same detector. There are also CO2 Detectors which sense for carbon monoxide.
There are also other multi criteria detectors which have photo smoke detection, heat detection, infar red detection etc all in the same detector.