SEO URL

URL in SEO , this means URLs of your website. SEO says `if your URL contents (-) dashes, then we split it into two words , and if your URL contents (_) underscores, then we join those words and index it. `
So for example if you have a URL to your website like this http://www.example.com/article/how_to_earn_money_online.htm then the last words of this URL i.e (how_to_earn_money_online) these words or pharse is indexed like a single word. So when any one search for this phrase then only your site page will appear , else it won`t. The better way of doing this is to use (-) dashes as a splitter in two word for example after using dashes your link should look like this http://www.example.com/article/how-to-earn-money-online.htm then google will index word `how` as a different keyword , then word `money` as a different keyword , then word `online` as a different keyword.
So it is better to use dashes over the underscores , but if you had already using underscores , then don't freak out don't change the URLs. If you are doing it fresh completely new website then it should highly recommended to use this (-) over that (_).