How do you know which computer files are legal to share?

When is a file legal to share?

In certain situations, it is legal to download, upload or otherwise share files that you find on the Internet. If a work is in the public domain, it means that the copyright on that work has expired and the work may therefore be used and shared freely [source: U.S. Copyright Office]. If you create a work and you still hold the copyright for that material (i.e., you have not signed over the copyright to an employer or agency), you may of course choose to share it as you wish. And works created under a sharing license such as Creative Commons or the GNU General Public License can be shared legally under the conditions specified by the creator of the work. Under a Creative Commons license, for example, a photographer may allow any non-commercial use of his or her photography, or an author may give Web site owners blanket permission to reproduce his or her blog posts, as long as proper credit is given [source: Creative Commons].

Unfortunately, it's not always easy to tell whether a file meets the above criteria. Original works published under Creative Commons or other shared licensing standard will often include a description of how the material may or may not be used. But if another Web site reproduces the licensed work without giving the required credit, you may download it without realizing that the site where you found the file is not the original source. And while most works published in the United States before 1923 are now in the public domain, it can be difficult to determine whether the copyright has been renewed on later works. As a general rule, assume that any recent creation -- and certainly any current song, movie or TV series -- is still under copyright and is therefore not in the public domain.

Luckily, there are steps you can take to ensure that the content you find online is legal. The Internet Archive is a non-profit organization that maintains an Internet library of works in the public domain, including movies, texts, audio files and software programs. The Recording Industry Association of America (RIAA) maintains a list of legal music sites like Pandora and Rhapsody that let you find and listen to copyrighted songs, and the Motion Picture Association of America (MPAA) maintains a similar list of movie and video streaming services, such as Netflix and Hulu.

You can also go directly to the Web site of a musician or television network to see if the song or program you're looking for is available there. Copyright law still prevents you from sharing or redistributing the file without the copyright owner's express permission, but in many cases you will be able to stream or download the content legally -- and for free.

But let's say you can't find what you're looking for through any legal channels, or you don't want to pay for the music you download. What's the worst that can happen if you share an illegal file? Read on to find out.