Humidors: When necessary, you can get away with keeping properly humidified cigars in a sealed plastic bag with a small, damp paper towel for a day or so. Left out in a heated or air-conditioned room, a cigar can dry out and die as quickly as the most delicate flower--in less than an hour. In a properly maintained humidor, the atmosphere inside of which closely mimics that of a tropical isle, cigars can be kept for years.
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. 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; it's better to have air circulating between the cigars in your humidor than it is to squish them in too tightly.
While a humidor needs a device that maintains moisture levels, it does not necessarily need a gauge. Some humidors, however, come with hygrometers, which indicate the interior humidity. While the analog models (the round gauges with a needle inside) often have the appealing style of a dial on a sports car's dashboard, they are frequently inaccurate. Digital hygrometers, on the other hand, are usually reliable to a level of plus or minus 2 percent.
No matter what a thermometer or hygrometer says, the true measure of your humidor's performance will be the condition of the cigars inside. If the cigars are exuding a little oil, the conditions are perfect. If they seem too dry, you add more water. If they turn moldy, you have to throw out the cigars (probably with a tear or two in your eye), no matter what the hygrometer says. There are even pesky critters which can bore through the contents of your humidor if the conditions are not right. See Cigar Problems.
Maintaining a Humidor:
Humidors are much simpler to maintain than other balanced environments, such as tropical fish tanks. All you have to do is keep the lid or door shut and periodically add distilled water to the humidifying device. (If you use regular tap water, the minerals in it are likely to collect on the humidifier and diminish its ability to emit and absorb moisture.)
A little common sense helps, too. Exposing a humidor to temperature extremes such as in direct sunlight or on top of an air conditioner or radiator is bad for the humidor--and your cigars.