A photoelectric sensor detects ultraviolet or infrared light emitted by smoke particles, and an ionization sensor uses a small electric current which smoke will disrupt.
In general, ionization alarms detect flaming fires faster than photoelectric alarms, while photoelectric alarms detect smoldering smoke more quickly. The difference in accuracy for varying fire types isn’t huge. Proper placement of a smoke alarm is the best way to increase effectiveness.
The information for this answer was found on our Carbon Monoxide & Smoke Detector Laws answers.