CNET recently posted an article on the science of smoke alarms. While most smoke detectors use either photoelectric or ionization sensors, Halo smart smoke alarms use both kinds of smoke sensing technology to increase the speed of detection for both slow-smoldering and fast-burning fires.
Ionization sensors contain a minute amount of a radioactive element, which carries a slight electric current, in an ionization chamber. When even a small amount of smoke enters the ionization chamber, it disrupts the electrical current and triggers the alarm. Because they're more prone to giving false alarms, smoke detectors with only these sensors are often turned off by frustrated home-dwellers.
Photoelectric sensors are better at responding to fires at their earlier, smoldering stage, before there are heavy flames. Photoelectric sensors contain a light source and a light-sensitive electric sensor (ie. photoelectric). Light misses the sensor under normal conditions but, during the early stages of a fire, smoke entering the chamber will scatter the light and sound the alarm.
Both kinds of sensors have pros and cons. While ionization sensors are inexpensive and require less power, they're more prone to false alarms. Photoelectric sensors are usually more expensive to install and keep powered, but have fewer false alarms. There's also a measurable difference how quickly each detects different types of fires and, in the event of a housefire, quick detection can significantly affect the amount of damage done.
For these reasons, fire experts recommend using alarms like Halo, which incorporates both photoelectric and ionization sensors and offers the most protection.