Personally, this was a confusing subject for me a couple of years back. Should I use an underscore… or a dash? This video will help you understand the differences.
Choosing the right URL for your website can be a challenge at times and so can naming additional pages within your website. So when should you use an underscore (_) and when should you use a dash (-)?
Matt Cutts, Google’s head of Web Spam Team, takes us through an easy to understand video explaining the differences between dashes and underscores, and more importantly, how Google views each.
Google views underscores (_) in a URL as a “combiner.” This means if you separate several words with underscores, Google will view them as related to each other and index them together.
Google will see dashes (-) as a “separator” and will index the two words separately.
Matt Cutts advises, however, that if you already have a site up and running you should probably leave the URLs as they are. Only when developing a new site should you probably worry about the impact of dashes vs. underscores.