A fireplace blower, part of a fireplace heater system, is a device designed to spread warm air from a fireplace more evenly throughout one or more rooms of a house.
Without some way to force heated air away from a fire, whether it uses wood, gas, or some other fuel, the bulk of heat that is not lost up the chimney is simply radiated into the surroundings. Radiated heat from a fire is only effective at short range. A blower, or some other kind of fan, is essential for distributing warmth from a fire more evenly to all parts of a room or series of rooms.
Various kinds of blower are available to suit different makes and types of fireplace. Those that work with fireplace inserts, for example, are not suitable for zero-clearance fireplaces. Some fireplaces come with blowers already installed; in other cases, you will have to fit your own or have it installed by a professional. Ease of installation depends to some extent on the availability of a suitable, nearby power supply. If such a supply isn't already on hand this will add to the set-up cost. Be careful about matching your fireplace and requirements to a particular blower or fan system.
Some more points to consider
A blower won't affect the rate of combustion of a fire or the quality of fire produced. Its sole purpose is to move air that has already been heated in the space around a fire into a bigger volume. Blowers are particularly effective in large rooms.
Consider the features that are offered with a blower. These typically include a thermostat that can be set so that the blower kicks in and shuts off at predetermined temperatures. A dial control is handy for adjusting the speed of the blower and for manually switching it on or off. Be aware that some owners complain of their blowers being too noisy, so look for quiet models.
Also note that a blower placed directly alongside or in front of a fireplace may be effective in driving warm air to all parts of a room, but is not so useful in distributing heat to other parts of a house. To move warm air out of the room in which a fire is located into the rest of a house, consider installing a doorway fan. These mount at the top of a doorway (preferably on the wall opposite the fireplace) and suck warm air out of the heated room and into other rooms and passageways.
You might try using a combination of fans to achieve optimum distribution of heat throughout the rooms and levels of your house: a fireplace blower, a doorway fan, a ceiling fan, and, if you have a forced-air heating system as well, a furnace fan (with the furnace burners turned off).