A fireplace is just an open fire box basically where you put some logs on it and start a fire.
Fireplaces are nice for the look but they're very inefficient way of heating your home with wood. Most of the heat gets sucked up the chimney.
A fireplace insert is basically a wood stove that you insert into your open fireplace and it usually has a blower on it where it will blow the heat into the room.
The fireplace insert puts out more heat since the fire is in a metal box and lets the heat radiate out of the fireplace insert just like a regular wood stove.