When it comes to creating a mobile-friendly site, there are a number of approaches that you can take. In fact, each of these approaches is designed to create a web presence that is mobile-friendly. There are basically 3 types of approaches. Each of them provides unique end user experience.
Responsive: In this method, the page, the HTML, the URL, the images and every other aspect of the web page will remain the same. Only the CSS will give different rules, depending upon the width of the browser.
RESS /Dynamic/Adaptive Serving: Here the URL will remain the same. However, the server will send different CSS and HTML, depending upon the nature of device that is requesting for the page.
Separate Mobile Site: This is generally hosted on a typical subdomain or at times, on a subfolder. However, there are occasions when it uses the same URLs as of any desktop website. It serves a different HTML to desktop and the mobile visitors, which is based on the user agent. This is termed as Dynamic Serving.
However, since it is an altogether different set of web pages, the options here are limited. Hence, the user can opt for an altogether different structure of the site. In that case though, the mobile URLs might necessarily be different.