Fireplace doors are a way to add a decorative element to your fireplace, but they are also a good idea for safety reasons, as they keep sparks from flying out of the hearth and landing on carpets and such. They also safeguard pets and children from getting too close to the fire. Glass doors can contribute to the efficiency of your fireplace, too, by keeping warm air from escaping up the chimney.
Here is some information to keep in mind when shopping for them:
Glass fireplace doors should be made to precisely fit the firebox opening. (If you're buying a new unit, most manufacturers of prefab fireplaces will offer doors as part of the sale.) For standard-sized fireplace openings, arched or rectangular glass doors usually work. The glass can come clear, smoked, or tinted, but it should be tempered. The best fireplace door frames are made out of brass or steel (these are sturdiest); cheaper models may be made from aluminum. There are a wide variety of finishes for the frames, so the choice is up to you. Examples include antique brass, copper, brushed steel, and nickel.
Just to confuse you further when you're trying to decide, here's a quote from a book on fireplace design:
"Doors may be cabinet-style, bifold, or full-fold; models with no center bar give a clean, all-glass look and a full view of the fire. Some glass doors come with integral wire-mesh curtains, and some have top and bottom louvers for fire control."
Source: Ideas for Great Fireplaces