With so many different doors on the market, the fist step in any door installation is deciding which type of door is going to be the best fit for your needs. Here's a quick primer on door types and common uses to help you get your door installation underway.
Door Types
Hollow Core Doors are lighter doors with a hollow, reinforced center. They are much cheaper than solid wood, steel, and fiberglass doors and are perfect for indoor applications. They won't hold up to weather, however, and provide little to no security, making them unsuitable for exterior use.
Solid Wood Doors can be used for interior or exterior applications. They are much heavier and sturdier than hollow core doors, but cost more as well.
Steel Doors are most often used as exterior doors. They are fire and weather resistant, and by far the best option when it comes to providing security for your home.
Fiberglass Doors can be used anywhere. These doors are popular because they are virtually maintenance free. They never rot, crack, rust or split, and are much easier to refinish than wood or steel.
Folding and Bifold Doors are perfect for interior closets, washrooms and the like. They swing on hinges or slide along runners and are far and away the easiest door to install yourself.
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Sliding Glass and French Doors are especially popular as exterior doors adjacent to patios, porches, decks, and yards. They provide excellent views, more light, and can be opened wider than single framed doors, though they generally aren't as secure.
Storm or Screen Doors are secondary doors added to exterior entryways. They are most often made of steel, vinyl, or aluminum and serve two purposes. In winter they provide an extra barrier from cold and inclement weather, and in warmer months they can be opened to allow air to circulate in through a screen.
Pre-Hung Doors
One other thing to look into when choosing a new door are pre-hung doors. A pre-hung door is pre-fit to the frame and all of the hinges and hardware are fit and attached before it ever leaves the factory. Buying a pre-hung door takes a lot of the work out of door installation because you won't have to mortise hinges into the door and jamb or worry about lining up the door properly before it's installed. All you'll have to do is install the frame, hang the door, and you're good to go.
Talk to the Pro's
Door installation is a doable project for an experienced home handyman, though it is important you get the job done right. If everything's not lined up and installed correctly you could end up with a wide range of problems down the road, including doors that stick, won't shut, and that don't seal properly and let in drafts that reduce your home's overall energy efficiency. Talk to a professional door installer about proper installation procedures, and don't be shy about calling them in to do the job for you if you think a door installation project might be over your head. 
Matt Goering, formerly a carpenter and house painter, is a freelance writer for the home improvement industry who has published over 600 articles.