which is better to add two names (-) or(_)

Question

hi when i write css or html i found that i want add two name like this

web-development
web_development

which one is better according SEO or write style name, file name or image name.

Answer 1

The first one is better. Also see this post by Google employee Matt Cutts: http://www.mattcutts.com/blog/dashes-vs-underscores/