Say you have a website page that is all about french holiday property. You will probably want to include these words in your page name so that it might make sense to search engines (and to any real person that likes to see where they are on your site).
It is better for you to separate each word with a hyphen (-) than it is to separate them with underscores (_). So, www.mysite.com/french-holiday-property would be better than www.mysite.com/french_holiday_property and www.mysite.com/frenchholidayproperty. When Google crawls your site, the use of hyphens (or dashes) enables it to read your URL properly, ie ‘french holiday property’ in our example, as it views hyphens as separators. On the other hand, Google cannot see underscores, so it will not see a separation between the words and will read ‘frenchholidayproperty’, which is difficult to read and to understand.
It might be a little tricky (but possible) to change the page names on an established site, but it is something worth bearing in mind for new websites, additional pages, and existing sites which haven’t already built up good pagerank and backlinks.