A permalink, or permanent link, is a URL that points to a specific blog or forum entry after it has passed from the front page to the archives. Because a permalink remains unchanged indefinitely, it is less susceptible to link rot.
You might be referring to a pretty permalink which is a question a lot of WordPress users ask all the time. The more keywords you have in your url, the better your post will rank in Google. The default value with WordPress is a number which doesn't do much good so I recommend changing it. Read more about it here: codex.wordpress.org/Using_Permalinks
Are you asking about wordpress permalinks? If so it makes use of mod rewrite so the url structure is more seo friendly because it's keyword rich.