I still see lots of URLs having underscores, so thought I’d discuss that a little. Who cares, right? Well, Google cares, for one. If you use an underscore in your URL like this: www.mysite.com/my_best_post.html – then what Google will do is join the words together when it gets indexed “mybestpost”. On the other hand when you use dashes: www.mysite.com/my-best-post.html – then Google will consider the words separate and index them that way.
Both ways will work, as proven by so many old sites still using the underscores. But the dashes are definitely better for your SEO health, and every little bit helps.