It's important that all homeowners recognize the difference between interior doors and exterior doors. Interior doors are, of course, only meant to be used inside your home. They aren't constructed to be secure or to provide any insulation.
Hollow core doors are the most common type of interior door. They are constructed of two pieces of thin veneer plywood, between which sit supports made of cardboard that keep the door rigid. Hollow core doors are often used in closets, children's rooms or any room where sound insulation isn't important. Even though these doors are ineffective sound barriers, they provide a flat, private surface to paint or stain for those on a modest budget.
French doors have multiple double-panes of glass running horizontally down the full length of the door that provide both sound and heat insulation. Traditional French doors are assembled with individual small panes of glass separated by mullions, which are metal strips placed at regular intervals along a window line.
Folding doors, also known as bypass doors, are typically made of hard plastic or vinyl, and fold like an accordion to the side of the doorway to allow entry where space won't permit the use of a swinging door. They often attach to the wall with a magnetic clasp. While folding doors provide a physical barrier, their flimsy construction doesn't provide any soundproofing, which is why they are most often used only for closets.
Mirrored doors are most typically used as closet doors because they allow you to check yourself out while dressing. Mirrored doors are also a fantastic way to give a cramped room the illusion of extended space or dark rooms a lighter, airy look.
Pocket doors are the most ingenious space-saving doors, ever. They retract into a pocket opening into a wall, so they don't take up any floor space. Pocket doors are typically built from wood but some have beautiful stained-glass inserts that add an elegant touch to a room.