If you want your SFDC dashboard to click through to something else other than the underlying report, an alternative URL can be specified. If the URL is another report in SFDC, the preferred way is to have /reportId as the URL. Using a server name free URL makes refresh to sandbox work without additional effort.
Sometimes you may simply user reportId as the URL. This will work when viewing the dashboard, because that's basically a relative path. However you need to remember that this WILL BREAK DOWN if the dashboard gets snapshot on the Home page. That's because the URL for Home is servername/home/home.jsp, which renders the relative path to an incorrect location. So, I'd almost use the dash version (absolute path) at all times to avoid this subtle issue.