But first, some legal advice: If you're planning to reuse content for business purposes, be on the safe side and consult an intellectual property lawyer who has internet experience.

Reusing content can be tempting. Being online can seem like you're a kid in a candy shop with your mother's credit card. The internet is the largest repository of data ever compiled, available at your fingertips.

In all likelihood, any content you come across is copyrighted, which means it's illegal to copy it without the owner's permission.

Exceptions include public domain content produced by the government with tax dollars and certain older works such as the plays of William Shakespeare.

According to U.S. copyright law, new creative works are copyrighted the instant they assume any tangible form, such as paper, film and the internet.

Works of intellectual property don't need to be accompanied by a copyright symbol or registered with the U.S. Copyright Office (www.copyright.gov). If you take these steps, however, you may be eligible for additional protection or rewards if you file a copyright infringement lawsuit.

U.S. copyright law does let people reuse the intellectual property of others for the benefit of society in certain situations, which is called "fair use." After all, the cornerstone of democracy is the free exchange of ideas, and our legal system promotes this in part through the fair use doctrine of the copyright law.

The key concept is "fair." As a general rule, it's more likely that you can fairly and legally reuse the work of others if your purpose is more for the common good than your own profit. For example, if what you're copying is factual rather than creative, if you copy a relatively small part of the work, or if your copying the work doesn't diminish the originator's ability to profit from it.

For details about fair use, you can check with the U.S. Copyright Office (www.copyright.gov/fair-use). Stanford University's fair use information (fairuse.stanford.edu) is a particularly good resource.

Copyright and plagiarism are related but different concepts. 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.

Plagiarism problems arise primarily in academic and journalism settings.

The devil is in the details here as well, since creative people going back to William Shakespeare and earlier have used others' work as inspiration for their own.

The internet also provides ways to determine if someone has lifted your work. 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. You go to the Copyscape site and type in the address of a particular webpage 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 keywords in a deliberate effort to be undetected, Copyscape can often still find it.

Other services also include tools to help keep people from filching.

Turnitin (www.turnitin.com) is targeted toward teachers, while iThenticate (www.ithenticate.com) is targeted toward publishers and research organizations.

If you discover another site copying your content, advice from experts is to first collect all the evidence you can 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.

Other tactics include contacting the web host or registrar as identified in your WhoIs search, the person's advertisers, and Google.