What is duplicate content? Duplicate content is exactly what is sounds like, multiple pages in which the bulk of the content is similar.
Two issues (of several) with this:
1. When google crawls your site it compares your content against other sites, if your site has copy that is a duplicate of another site typically the site that is older will get the credit, so by not creating your own unique content your effectively hamstringing yourself.
2. If you do this to often you may find your pages falling out of the index altogether by not providing unique content.
A great article about Duplicate Content and how Google handles it.
http://googlewebmastercentral.blogspot.com/2008/09/demystifying-duplicate-content-penalty.html
You may also want to read this post because duplicate content can affect your rankings but there are different types of duplicate content and spamming a page with many URLs can harm your site:
Why you should care…
http://googlewebmastercentral.blogspot.com/2007/09/google-duplicate-content-caused-by-url.html