Dash (-) or underscore (_) in your URLs?
For a URL / website address, having dash (or sometimes referred as hyphen) works better than underscore. There are two main reasons as to why dash is more preferred. Firstly, Google has stated in its Webmaster Central that it recognises dash/hyphen as a word separator. On the contrary two words joined by underscore are viewed as a combined word. Therefore from Search Engine Optimisation (SEO) point of view, it is recommended to use dash or hyphen over underscore for your URL if need be.
In addition, dash also holds the advantage from user-experience point of view. In a standard link, dash can be easily identified as a separator between words because of its position whereas underscore could easily blend with underline and being mistaken as just a normal space especially due to the fact that it usually sits just above the underline.
Have a look at these two URLs
As you can see, the one with dash is much easier to read and more friendly. Moreover Google does not have any guidelines regarding the use of underscore in URLs so why would you want to use it over dash.
Image credit Cubosh on Flickr