Hi there,
The post slug is the part of the URL after the date in a post’s URL. When the default post slug is created, all letters will be converted to lowercase, spaces will be exchanged with dashes, and any special characters will be removed.
You can modify the post slug by clicking the Edit button next to it. When you’re finished editing, click OK. After you are done making your changes don’t forget to click the Update button to update the post and save these changes.
This is especially handy if, for example, your page title is very long, but you would like a URL that people can remember more easily.
If your language uses a non-western character set, long post titles can break post slugs. To fix that, shorten the post slug to a single word.
Keep in mind that only the post slug can be changed. You cannot change any other part of the url such as removing the date.
I pulled this info from this page:
http://en.support.wordpress.com/posts/new-post-screen/
Does this answer your question, or are you asking about something else entirely?
Cheers!