For example, you could have Beatles albums listed by popularity or by albums on two pages:
To a search engine, these are two different pages. The major search engines will eventually learn that they contain similar content and prioritize one - but you can control this by adding a tag in the <head> section of both pages.
<link rel="canonical" href="http://www.example.com/beatles.php?sort=pop" />
This basically allows you to tell Google - "Out of all the pages with this link, prioritize this link instead in search".
For more information, check out Google's explanation of Canonical Pages.
For those of you using Wordpress, Blogger, or most other popular blogging software, this is added automatically for you.