A humidor is any container intended to store cigars at the appropriate humidity. 

Cigars are hygroscopic in nature, which means they will readily release or absorb moisture from the environment until they are in balance.  The optimal environment necessary to store cigars long term is 70% relative humidity (RH) and 70° Fahrenheit.  Some people prefer to smoke cigars kept at 65% RH or lower.  This is a matter of personal preference and it may vary for different cigars.  Ultimately, 70% RH is still ideal for long term cigar aging which could span years or decades. 

Storing cigars in a refrigerator or freezer will damage the tobacco and retard the aging process.  Conversely, keeping cigars at higher temperatures can lead to the hatching of tobacco beetles.  At higher temperature and humidity levels, the tobacco beetle larvae will hatch and the beetles will infest and eat through the cigars.  In addition, significant changes in temperature and humidity will cause expansion, contraction, hydration, and dehydration that will negatively impact the natural harmony and construction of a great cigar.  Consistency is ideal for proper storage.