Unless you come across an inferior window product, all storm windows offer some degree of protection. What you need to do is determine which kind will work best for your particular windows. You should also ask yourself why you need these type of windows in the first place because this will give you a good idea of which type you should purchase. Do you need them for sound proofing, do you wish to block a draft or do you just want to add an extra layer of protection to your window frames?
If you have traditional single-paned windows, more than likely they are not energy efficient. Even if the glass is pretty thick and sturdy, you still may feel a draft every now and again. If the glass is too thin, you may have an issue with too much noise as well. The best type of insulated windows for a situation like this would be triple-track storm windows.
They are called triple-tracks because they have three sections that hold the top sash, bottom sash and screen. You can easily remove each piece whenever you want to clean them. You can also raise the window up and down whenever you feel like letting natural air into the house.
If you wish to protect a thin piece of bathroom glass or a leaded glass window, you can purchase fixed storm windows. Fixed storms are stationary pieces of glass surrounded by a frame. They are usually pretty durable and some of them come with a light tint for energy efficiency. Once you put a fixed storm over your existing window, you will not be able to remove it easily. They were not meant to open and close like triple-track storms. Their sole purpose is to provide permanent protection for a window that may be too old or too thin.
If you want the least expensive alternative, you can install magnetic storm windows. These storm windows are usually made out of plexiglass or some sort of plastic. Their lightweight makes them easy to handle, replace, and remove when necessary. Magnetic storm windows are for the interior of the home. Their main purpose is to block out wind. They do very little for sound protection but if they have a slight tint, they will protect furniture and rugs from excessive sunlight.
It is best to have a professional contractor install these windows. This is especially true for fixed storm windows because they tend to be heavy and awkward to install. However, if you feel comfortable as a do-it-yourselfer, you can order your storm windows online or at any department store. Custom-made magnetic storm windows may not be readily available so you may need to order them from a specialty window manufacturer.