As a huge repository of information, photos, music, videos, and other content, the Internet is a huge boon to society. We have available at our fingertips knowledge and entertainment that previously took far more time and effort to find, if we could find it at all.

Many of us--individuals, businesses, nonprofit organizations, educational institutions--put content on the Internet out of the goodness of our hearts or to benefit in some way, perhaps through ad revenue when people access our sites, blogs, or YouTube videos.

What about when others reuse your content? The devil, they say, is in the details.

First, about copyright. According to U.S. copyright law, all creative works are copyrighted the instant they assume a tangible form, whether on paper, film, or on the Internet, meaning they have legal protections against others copying them.

Works of intellectual property don't need to be accompanied by a copyright symbol (a "c" within a circle) and don't need to be registered with the U.S. Copyright Office (www.copyright.gov), though taking these steps can make you eligible for additional protection or rewards in the event of a copyright lawsuit.

Second, about fair use. U.S. copyright law allows people to reuse the intellectual property of others for the benefit of society under certain limited situations. Examples include reusing small parts of others' works when commenting on or criticizing them.

For details about fair use you can check with the U.S. Copyright Office (www.copyright.gov/fls/fl102.html). Stanford University's fair use information (http://fairuse.stanford.edu) is particularly good.

Third, about plagiarism. Copyright and plagiarism are often confused, related as they are. Copyright involves copying the form of the work of someone else, such as the words or video. Plagiarism involves claiming someone else's ideas as your own even if you don't copy the words or other forms used.

Plagiarism problems arise primarily in academic and journalism settings. The devil is in the details here as well, since creative people going back to Shakespeare and earlier have used others' work as inspiration for their own.

Among the things you also can't do, legally, with the copyrighted works of others is claim them as your own. This doesn't stop people on the Internet from doing this every day.

You can determine if someone has lifted your work in a number of ways. Often a simple Google search using a particularly distinctive phrase or other string of words can turn up instances of unauthorized copying.

Another tool is Copyscape (www.copyscape.com), a web service specifically designed to track web infringement. It's available in free and pay versions.

With the free version, you go to the Copyscape site and type in the address of a particular web page of yours. Leveraging Google's technology, it searches the web for pages with words copied from yours. One of its tricks is that even if another site moved sentences around or changed some key words in a deliberate effort to be undetected, Copyscape can often still find it.

The premium version of Copyscape, costing five cents per search, lets you copy and paste content to see if it's located elsewhere on the web. This can be helpful if you buy content and want to verify that it's original.

A further beefed-up version of Copyscape, called Copysentry, lets you set up automatic daily or weekly searches for copyright infringement, emailing the results to you. It starts at $4.95 per month.

Other services are also aimed at keeping people from filching others' work. Turnitin (www.turnitin.com) is targeted toward teachers, while iThenticate (www.ithenticate.com) is targeted toward publishers and research organizations. Both are available under various pricing schedules.

If you discover another site copying your content, collect all evidence to support your case. Next, find the owner of the site. Most sites include contact information. If not, do a WhoIs search--type "whois" into Google to find WhoIs servers.

Consider sending a polite, nonbelligerent email asking the person to remove your content. Another tactic, if the infringement is blatant, is to email the person an invoice.

If that doesn't work, consider emailing a more formal cease and desist order. A Google search for "copyright cease and desist order" will turn up templates and examples. If that in turn doesn't work, you can further ratchet things up by contacting the web host or registrar as identified in your WhoIs search, the person's advertisers, and Google (http://support.google.com/bin/static.py?hl=en&ts=1114905&page=ts.cs) and other search sites.

Finally, you can hire an intellectual property lawyer with Internet experience to do some or all of these things for you.