Path Module

The Path Module is a Drupal core module. Enabling it allows you to create URL aliases.

Drupal's standard URLs (once you have enabled "clean URLS" in the Admin panel) are in this format:
http://example.com/node/25

Once you have enabled the Path Module you will be able to create URL aliases for each URL. If you created a URL alias for that URL (node 25) called custom-page-title, you would then be able to access the content of node 25 at http://example.com/custom-page-title.

You would also still be able to access the content of node 25 at http://example.com/node/25. Generally, you do not have to worry about this unless your site has already been indexed with the original "node" URLs. In either case you could install the Global Redirect Module which would automatically redirect http://example.com/node/25 to your URL alias at http://example.com/custom-page-title.

A related module is the recommended Pathauto Module which automatically creates URL aliases for each node on your site.