A fireplace insert is basically a stove without legs that is placed into an existing wood burning fireplace.
Essentially all fireplace inserts whether they be gas, pellet or wood burning are self contained units that can be installed in virtually any wood burning fireplaces and made to increase their efficiency without having to convert them to gas. They provide far greater heat than a wood burning fireplace with efficiency frequently in the 80+% range. They are essentially furnaces that happen to look like traditional fireplaces!
Wood burning inserts can be made of steel or cast iron, modern or traditional looking, virtually flush or protruding, simple or dressed up in a variety of ways with attractive surround options. They can have blowers (many come with blowers standard) to aid the circulation of the heated air in the room.
Sometimes wood burning inserts are used when the fireplace and / or chimney structure needs repairs and cannot be used in its current state. Since wood inserts have their own venting system and a firebox, they only need the existing fireplace to encapsulate them. Masonry and flue repairs cost sometimes almost as much (if not more) than buying and installing an efficient insert inside of a fireplace.
Wood burning insert venting diagram courtesy of Napoleon
Notes on Venting of Wood Burning Inserts
All wood inserts require that the venting used vertically terminate through the existing chimney. The venting liners can be flexible or rigid, stainless steel or aluminum; the choice of venting will solely depend on the manufacturer's requirements, model of the insert, diameter of the existing chimney and its height.