first one is simply a post id or a normal url, that the search engine indexes simply. but the second one is a Canonical or SEO friendly or scribed URL or Re-written URLs that SEs will index as it is and that really is of Great value in indexing your content.
If you would like to create Friendly URLs for your site (much like the URL for this thread: http://www.webdesignforum.com/9786-what-does-term-mean.html), I have built Friendly URL generator which will create the necessary .htaccess file for you.