How are fireplaces and inserts different? Is also used to convert a wood fireplace into a gas fireplace?
A fireplace is the broad term for the place you can safely have a fire. Be it gas or wood. A gas fireplace was designed to emulate the look of a natural wood burning fireplace. Now they come in a wide variety of styles and shapes. A wood insert is designed to contain a fire and allows for control of the flame by controlling the air entering the combustion chamber. A gas fireplace insert is designed to be installed in an existing fireplace opening and usually mimic the look of a wood burning fireplace insert.
I had the same question about a year ago when I needed to have a fireplace replaced. The store owner explained that in current terminology, a fireplace insert is designed to be installed in an existing masonry fireplace. It seals up the combustion chamber to provide higher efficiency than an open firebox. It does not provide thermal insulation, which is why it has to be installed in a masonry fireplace.
A fireplace can refer to either a conventional masonry fireplace, or a modern zero-clearance device that can be installed in a wooden structure. They include enough insulation that they can be installed directly against wooden materials.
Both fireplaces and fireplace inserts can be either wood-burning or gas fired.