There are three types of smoke alarms, ionization, photoelectric and a combination of the two which is commonly called a "dual". Look for the UL stamp on any smoke alarm.
Dual smoke alarms combine ionization and photoelectric sensor systems to enhance home safety. Recent research has shown that the standard ionization alarms have been unreliable in multiple tests of slow, smoky fires. Ionization alarms perform best in fast flaming fires. A combo unit is considered to provide greatest overall safety in either situation
Ionization smoke alarms monitor "ions" or electrically charged particles. Smoke particles change the electrical balance of the air. The alarm will sound when the change in electrical balance reaches a preset level. (IFSTA Fire and Life Safety Educator Page 38.)
Photoelectric smoke alarms use a beam of light and a light sensor. Smoke particles change the amount of light that reaches the sensor causing the alarm to sound. (IFSTA Fire and Life Safety Educator Page 39.)
Heat detectors are best used over hazards where flaming fires could be expected such as a garage or utility area. Heat detectors have a slower response than smoke detectors according to the National Fire Protection Association because heat generated by small fires tends to dissipate fairly rapidly. (NFPA Fire Protection Handbook Page 5-20, 5-58 and 59.)