Great question BTK......
Here is a link that answers many questions including the ones you asked from the U.S. Fire Administration;
USFA Smoke Alarms
Basically, they suggest replacing them every 8-10 years. I recently upgraded the ones in my home to the newer dual-sensor photo-electric and ionization detectors. I bought mine from Amazon.
Without getting into details too much, the dual sensor detectors "detect" different types of fires. The innards of a dual sensor smoke alarm include a photoelectric sensor and an ionization sensor. Photoelectric sensors are very good at detecting slow, smoldering fires like fires which start in mattresses, couches, carpeting, and drapes. An ionization sensor can detect a sudden burst of flame like a grease fire. The combined sensors work together to alert people to smoky conditions and fires.
The photoelectric smoke alarms in dual sensor smoke alarms use beams of light to detect smoke and fires. Under normal conditions, a light emitting diode creates a stream of light which can be read by the smoke alarm's sensor. In smoky conditions, when particles begin to obscure the beam, the sensor emits a signal to indicate that the beam of light has been disrupted, indicating smoky conditions or large amounts of particulates in the air.
These should NOT be confused with the combo Smoke Detector- Carbon Monoxide detectors- Those are entirely different.
Hope that helps!
Frank D'Angelo, ACI