the act of having content that is the same on two or more different web pages.  Duplicate content can be caused by canonicalization where you have different URLs for the same page.

Search engines may penalize a site for having too much duplicate content. Duplicate content is considered spamming by search engines and therefore should be avoided.  Things to avoid:

• Multiple versions of your homepage
• Multiple versions of the same page with different URLs
• Sub-domains with duplicated content
• Microsites with duplicated content
• URLs for the same page that are appended with unique IDs for tracking purposes (if the content is the same, it will be considered duplicate content by the search engines)

In order to prevent duplicate content from occurring consider:

• Block Duplicate Pages from Being Indexed - Use a robots.txt to do this
• Use Proper Redirection - ideally you should use 301 redirects any time you have to redirect a webpage.
• Be consistent with interlinking of site pages
• Use Top Level Domains - this specifically pertains to country specific content
• Be Smart with Syndication - if you are syndicating your content, be sure that there is a link to the original

Google Webmaster Guidelines on Duplicate Content