What's the difference between <pop: slug/>, <pop:permalink/> and <pop:url/>?

<pop:url/> will output the absolute url
Example: <pop:url/> = http://yourdomain/yourlink

<pop:permalink/> will output the relative path
Example: <pop:permalink/> = /yourlink

<pop:slug/> will output just the part of the path that is specific to the entry
Example: <pop:slug/> = yourlink

In general, for most cases <pop:permalink/> is the best choice though there are times where <pop:url/> or <pop:slug/> may be appropriate.