There are two basic types of detectors, photoelectric and ionization. Photoelectric units use a beam of light and a light sensor to detect smoke. The light is constantly on but is not pointed directly at the sensor. When smoke enters the detector light is reflected off the particles of smoke and onto the sensor. This sounds the alarm.
Photoelectric detectors respond faster to the large smoke particles created by smoldering fires.
Ionization units measure the electrical current in the air between positive and negative charges of ions. A small piece of radioactive material creates a low electrical current in the air that flows through the sensor. When smoke enters the detector it changes the current in the air between the ions. Thus sounding the alarm. Ionization units respond faster to the small smoke partials of fast flaming fire .