Today, if you want to buy or rent a movie, you have a lot of options. You can drive or walk to a store, pick up a movie and return it when you're done. If you'd rather not make the trip, you can sign up for a service like Netflix or Blockbuster Total Access and have your movies delivered to you. If you don't want to wait for the mail either, you can order and download movies online. In most cases, you can begin watching within minutes of starting your download.

Until recently, most people who downloaded movies online were using peer-to-peer file sharing programs, like BitTorrent and Kazaa, to share pirated videos. This usually violated copyright laws, but now there are lots of legal businesses that offer movies for sale or rent online. Terms of use, system requirements and selection of titles can vary widely from one to another. Similarly, different sites can use different protocols and applications to deliver movies. However, most movie download businesses have the same basic structure and operate using similar principles.

Amazon Unbox is a service that allows people to buy and download movies.

The most visible component is the Web site where you shop for movies. Most movie download sites organize their titles much the way retail sites do. You can search for specific titles, browse different genres or use ratings and reviews to help you decide which movies you want. Once you make a decision, some sites require you to use a download manager to get your movies. This is a program that can add your movies to a queue, resume paused downloads and keep track of which movies you've selected and paid for. Other sites require you to download and install a specific video player. For example, Vongo requires you to use its software rather than your Web browser to find and download movies.

Vongo software acts like a browser and a download manager, and it lets users set preferences for file management and connection.

Before buying movies, you typically have to set up an account at the Web site you've chosen. Some sites, particularly those that offer adult content, allow you to set parental control preferences. A few allow you to subscribe to a monthly download service rather than -- or in addition to -- paying for movies one at a time. Often, you can add favorite movies or genres to your account preferences, and the site software uses this information to make recommendations for you.

Here are the basic steps involved in buying and downloading a movie:

1. Log into your account.
2. Choose the movie you want to rent or buy. Depending on the site's setup, you might place it in a shopping cart or go straight to the purchasing options.
3. If applicable, the Web site's software checks your parental control information to make sure the movie complies with your settings. If necessary, the site can check your subscription information to make sure you have not used all of your allotted downloads.
4. The Web site stores your choice in a database. The site's software uses this information to make recommendations to you and to determine royalty fees owed to the company that owns the rights to the movie.
5. Billing software determines how much money you owe and presents you with a final total.
6. The site encrypts the details of your financial transaction so other people can't see it.
7. The site's content delivery system starts your download and notifies you when you can begin watching. Depending on the site's setup, you can download a file or view your movie as streaming video. When using a streaming format, the site sends the movie to your computer as a stream of information, and your player decodes it as you watch.

Exactly what you can do with your movie once it finishes downloading depends on where you purchased it. Some sites let you download movies on a rental basis, giving you 24 hours from the time you start watching to finish your movie. Your player, download manager or other software then removes the movie from your hard drive or makes it unplayable. Most sites also allow you to purchase movies, which are then yours forever.

How you can use those purchased files also depends on the site's digital rights management (DRM). This is one of the things you should pay attention to when choosing a download service. We'll look at DRM and other things to keep in mind in the next section.