Search engines treat dashes and underscores differently from one another. Google has clearly stated that when it comes to URL structure, using hyphens rather than underscores makes it much easier for them to identify what the page is about. Take a look at an excerpt below from the Google support blog on URL structure.
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Google Claiming that Hyphens are Preferable to Underscores in URLs

Senior Google Engineer Matt Cutts clearly explains in this Google Webmaster Help Video about underscores in URLs that hyphens are used as word separators while underscores do not specify any function. Search engine bots have a different way of interpreting your punctuation when crawling and indexing sites. Search engines have not been programmed to interpret underscores the way that we do. This difference in interpretation is not only limited to URLs but also applies to image alt tags.

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For example, if your URL includes tipsforinstantweightloss (with underscores) search engines read it as tipsforinstantweightloss. Obviously someone typing in these words would include spaces. Conversely, when you use hyphens to link a keyword in your URL; tips-for-instant-weight-loss, search engines can return the words in various combinations, as follows:

  • Tips for instant weight loss
  • Tips for weight loss
  • Instant weight loss
  • Weight loss
  • Tips
  • Weight
  • Loss
  • Instant
  • Tips-for-instant-weight-loss
  • Tipsforinstantweightloss

So, the probability of your website being shown in the SERPs is lower when underscores are used as opposed to when hyphens are used. If you are not bothered about optimizing your website for search, here are some reasons why hyphens in URLs are also preferable for people.

If your URL contains underscores the link will look similar to this:

http://www.tipsforinstantweightloss.com (http://www.tipsforinstantweightloss.com)

Whereas if your URL contains hyphens, the link will look similar to this:

http://www.tips-for-instant-weight-loss.com (http://www.tips-for-instant-weight-loss.com)

A user may mistake the underscores for spaces, as the underlining in the link hides the underscores. On the other hand, hyphens are clearly visible, so users are more likely to remember to type them. So, the use of underscores in URLs impacts usability as well as SEO.

Google will still crawl and index URLs that already contain underscores and it is not necessarily advisable to change your URLs if they currently contain underscores. As long as you have other ranking factors working well, you should have no problem ranking high in the SERPs. For instance, look at the screenshot below of a Wikipedia URL for the term cloud computing. It uses underscores, and yet Wikipedia takes the top spot in search results for almost all keywords.

Wikipedia URLs Use Underscores

As seen in the Matt Cutts video mentioned above, Google says that they will begin working out a way for search engines to interpret underscores in URLs as separators once they have finished modifying the other high-impact search ranking signals they are currently working on. The general advice remains, however, that if you have yet to choose a domain name, do not use one with underscores, and if you are building inner pages on your website, make sure your URL structures contain hyphens rather than underscores.

If you already have a website URL that uses underscores and its SERP rankings are not improving, you can use 301 permanent redirects to a URL with hyphens. For example, if your old URL is http://www.yoursite.com/old_page.html 301 redirect it to http://www.yoursite.com/new-page.html. You do not need to do this, however, if your website fairs well in the SERPs, as 301 redirects can reduce a bit of the link juice that you obtain by building links to your site.