Both humidors and coolidors are essentially airtight containers in which to store cigars at a constant humidity. The main differences are that Humidors are purpose built and usually from wood. Coolidors are cooler boxes that have been adapted for the storage of cigars.
Coolidors are cheaper than humidors and are a value for money way of storing large quantities of cigars. A lot of people have a coolidor where they store boxes of cigars for aging, whilst still having a humidor where they keep the cigars that they want to smoke.
"I drink a great deal. I sleep a little, and I smoke cigar after cigar. That is why I am in two-hundred-percent form." - Winston Churchill