What is an Arc Fault Circuit Interrupter?
AFCI's detect patterns of current that usually indicate some sort of electrical problem. Unlike a GFCI which is making sure that current going out = current coming back, AFCI's try to look for electrical usage that would not trip a GFCI but might be something causing an electrical fire. Most code now requires them for bedroom circuits.

The one major drawback for them is that some stuff tricks them into tripping. Often times larger machines with motors, like a vacuum cleaner or power tool can trip them. They are still a good thing to have, just be aware of this limitation