The biggest advantage of having glass doors on your fireplace is in being able to close it up when not in use, thereby preventing your central heat and air conditinoing from going up the chimney.
There are 2 different types of glass doors for fireplaces. Those made for a masonry fireplace (all brick) and those made for a zero clearance firebox (also called a pre-fab fireplace). Masonry doors are designed to overlap the opening of the fireplace but can also be custom made to fit inside the opening. Zero clearance doors are made to fit inside the opening. It is necessary to have the brand and model of the fireplace to make sure the door is right for your fireplace. This information can be found inside your fireplace on a metal tag.
Custom doors are also available for zero clearance and are manufactured in a large selection of frame finishes, giving your fireplace a designer touch. Doing a custom door will also allow you to have an overlap frame which will give a finishing touch for marble or tile fireplaces.