A humidor is, quite simply, a storage container designed to allow controlled air flow and equipped with a device that maintains the internal humidity in the range of 70 to 75 percent; its internal temperature should be maintained in a narrow range of about 68 to 70 degrees Fahrenheit. (Without something to maintain the internal humidity, it's not a humidor; it's just a box.) Humidors come in all sizes. Travel-sized humidors hold just a few cigars; room-sized humidors hold thousands of boxes of cigars.
Note that a humidor is not a sealed environment. Inside an airtight moisturized container, cigars are likely to become moldy. For that reason, it's better to have air circulating between the cigars in your humidor than it is to squish them in too tightly.