Dashes versus Underscores in Your URL
Back in 2008 in a post about best practices for URL structure, we alluded to our preference to separate words in URLs with dashes or hyphens instead of the very common;l used underscore. We wrote that underscores (www.domain.tld/red_widgets.htm) cause search engines to see the words in the page name as redwidgets, but if you use dashes (www.domain.tld/red-widgets.htm) the search engines recognize and index the two separate words, red and widgets.
Google has just released a video explaining the historical rationale for that, why it’s still true, and why it’s unlikely to change in the foreseeable future. Watch the explanation by Google’s Matt Cutts.