A space heater is a small heating unit, usually only designed to heat a single room.
Like other heating devices, a space heater carries the risk of starting a fire or creating a burn. A high proportion of fires started by space heaters result in fatalities. Many of the fires are started by flammable things placed too close to the heater. Certain kinds of space heaters can produce carbon monoxide, which can be dangerous, especially in enclosed areas.
In general, it takes less energy to heat a room than an entire home. So by turning down the heat and using a space heater in the room where the residents are, you may save energy. On the other hand, space heaters are not designed to heat large spaces, and they are generally not an efficient way to heat more than the area they were designed to warm. Also, if one part of a home is significantly cooler than the rest, a space heater may be a way to make that space warmer without overheating the rest of the home.