Masonry fireplaces typically burn wood. Prefabricated fireplaces more often burn gas (although wood inserts are by no means uncommon). Gas fireplaces are simple to light, relatively easy to clean and come in two varieties, vented and vent-free. Vented gas fireplaces are exactly that — fireplaces that ventilate exhaust via either a chimney (or, in gas fireplace parlance, a top-vent) or a direct vent, which runs vertically to the outside of the house. Vent-free gas fireplaces, a comparatively recent invention, obviate the need for ventilation by burning gas at exceedingly hot temperatures, resulting in nearly complete fuel combustion (although we stress nearly; vent-free fireplaces do emit carbon monoxide and cannot be left to burn indefinitely).
Gas fireplaces burn two types of fuel: natural gas and propane. The effective difference between the two, qua fire, is negligible (although natural gas does burn more “cleanly” than propane, leaving less residue in its wake). Rather, what truly matters in choosing between a propane fireplace and the natural gas variety is — location. For city dwellers, whose appliances are usually connected to their city's natural gas works, a natural gas fireplace is the logical choice. For those residing in rural areas, where access to natural gas can be precarious, it's frequently — although not always — cheaper to use propane. The key is to first of all check with one's local gas suppliers, whose inventories and rates can sometimes differ significantly.