This is a question very frequently asked.... Whether to use dashes or underscores when registering the domain or naming the pages in the website I suggest while naming the pages use dashes instead of underscores When you name your page as website-development.html SE will consider it as website development and when you name your page with underscores website_development.html, it is will consider as websitedevelopment
I don't think that underscore will mean that the keyword is what you have stated. Firstly the best thing is to get simple domain which do not have any dashes between them as this is quite easy to remember by you as well as your visitors. In case you do not get that kind of domain then it is better to acquire dashed domains.
I read a recent article about how only dashes separate words. Underscores still technically don't serve that function, so search engines separate them naturally as they would with a compound word not separated (i.e. compoundword = compound_word, but does not = compound-word). The other way to think about it is that Wikipedia uses underscores
Domain Name: While getting a domain name your first priority must be to get domain without dashes. Second priority must be Domain name with dashes. Underscores in domain name are not allowed so no question about that. Page Names of your Website While naming your pages first priority must be to use dashes. This is easily readable, looks good and is suggested by most of the SEO experts. You can use names without dashes if you like, but never use underscores. Search engines don't treat names with dashes and without dashes different so there is no specific advantages of using one over other.
for domain names both should be avoided. for page names dash is more user friendly. for seo I dont think it matters.
Matt Cutts recommends dashes: "I often get asked whether I’d recommend dashes or underscores for words in urls. For urls in Google, I would recommend using dashes." http://www.mattcutts.com/blog/dashes-vs-underscores/ That's a good enough source for me.
I've always used dashes, but I would always try not to have to do that, and get a domain without that in there as some people can't remember the way the domain is.
That's a very good question... if you want to make your url SEO friendly then using underscore(_) b'coz google cound underscore(_) as a keyword phrase and dash(-) count as a separate words....
According to Matt Cutts of Google, hyphens (dashes) are more SE friendly than underscores. And this issue is previously defined by Cutts early in 2009 I think. Anyway, no limits in learning... Enjoy eIDSeller