What is duplicate content?

According to Google Webmaster Guidelines;

"Duplicate content generally refers to substantive blocks of content within or across domains that either completely match other content or are appreciably similar."

Reference:

http://googlewebmastercentral.blogspot.com/2006/12/deftly-dealing-with-duplicate-content.html
http://www.google.com/support/webmasters/bin/answer.py?hl=en&answer=66359

Related SEO Question:

Few pages of my website have duplicate content, does it affect in SEO?
My website has 1000s of pages but getting less traffic.