Permalinks are the permanent URLs to your individual weblog posts, as well as categories and other lists of weblog postings. A permalink is what another weblogger will use to refer to your article (or section), or how you might send a link to your story in an e-mail message. Especially when they are used to link to individual postings, once a story is posted, the URL to it should be permanent, and never change.
Which one is better dashes or underscores in permalinks (URLs) for search engine optimization? I have got the answer: dashes are better than underscores! Go find the explanations from Matt Cutt, one of Googles famous Webmaster-liasons.