What is a Page Slug?

Many people ask what is a post or page slug? At WordPress.org you will find a great definition.

A slug is a few words that describe a post or a page. Slugs are usually a URL friendly version of the post title (which has been automatically generated by WordPress), but a slug can be anything you like. Slugs are meant to be used with permalinks as they help describe what the content at the URL is.
Example post permalink: http://wordpress.org/development/2006/06/wordpress-203/

The slug for that post is “wordpress-203“.