Humidors

This is one of those "GOD WHAT A NEWBIE!" topics. Unfortunatly , i have to ask it. What is a humidor? I know its a box (usually wooden) that keeps the cigars humidified to the right temperature
for smoking. But how do they work...and how do you set up a new one? How long does one last?

Thanks , it was VERY helpful. I figured thats how they worked...just wasnt positive.