URLs: Should I use hyphens, underscores or plus symbols?

Although there seems to be less difference now than there was in the past, hyphens are recommended if you want each of the terms in your URL recognized as an individual word.

The reason for this is that search engines find it easier to treat hyphens as word separators, just as they are in the English language. Underscores, however, are not normally used in English as word separators. In many cases it is desirable to see them as part of the characters before and after them, for instance when underscores are used in function names for programming, and other similar situations.

There are a couple cases where this might matter. For one, if someone uses the full URL as the anchor text for a link, then the search engines will be able to pull out those terms as relevant words. For another, it is possible that search engines use words in the URL as correlative factors to determine the relevance of those terms to the page content.

Matt Cutts has a dated piece that explains this, and there is a more recent Google Webmaster Help video from August 2011 saying that Google still does not treat underscores as word separators. You may as well make life as easy as possible for the search engines, for the same reason that you give them valid HTML.

If you are starting a new site, you are definitely better off using dashes.

Here is your answer:

https://webmasters.stackexchange.com/questions/374/urls-should-i-use-hyphens-underscores-or-plus-symbols

The Stack Exchange network of sites are highly ranked with Google and The Stack Exchange uses hyphens in their URLs.

This article by Jeff Atwood always clinched it for me: Of Spaces, Underscores and Dashes:

This_is_a_single_word, but this-is-multiple-words.

... though this no longer seems to hold in the case of Google (try searching for "web-site": it is considered as one word).