In its simpler form (sometimes called cigarette tin) it is an approximately cigarette-length flat metal box opening flatwise symmetrically on hinges into two halves, each storing one row of cigarettes, often held in place by a spring or an elastic strap. Typical cigarette tins in the United States of 1920s-1930s stored 50 cigarettes, hence their name "flat fifties" at the time.
Within smoking culture, cigarette cases may be fashionable accessories. As such, they may be made of precious metals, adorned with artistic engravings, monograms and jewels. Peter Carl Fabergé, while most famous for his Fabergé eggs, also manufactured exquisite cases of gold and gems for the family of the Tsar, some of which, (e.g. those owned by Danielle Steel) are reportedly worth up to $25,000 and appreciating. Alternatively, they may be leather-covered. Cigarette cases are also collectible items.
Cigarette cases used to be popular with soldiers, and many World War I and World War II veterans (e.g., the article about James Doohan) claimed that cigarette cases saved their lives by stopping bullets.
In 2003 the European Union witnessed a surge of cigarette case sales, attributable to the introduction of prominent black-bordered warning labels on cigarette packs, e.g., "Smokers Die Younger", etc., by an EU directive in January 2003. The cases were to get rid of warnings, with another way being various funny stickers, such as "You could be hit by a bus tomorrow".
The United States Census Bureau, for the purposes of industry statistics, includes manufacturing or adorning of cigarette cases into the category NAICS 339914 "Costume jewelry and novelty manufacturing".
Due to their compactness, conveniently fitting in a pocket, cigarette cases are often used to store or conceal various things. For example, quite a few James Bond's gadgets are based on cigarette cases, see, e.g., what Francisco Scaramanga's golden gun was made of.