There used to be a time when planning a mobile optimised website meant having 2 physically seperate websites, thankfully thats no longer the case. With the advent of HTML5 and CSS3 we can now dynamically change the layout and style of a website entirely based on screen size, type of device, or a devices capabilities.

For instance when a desktop user looks at a website navigational links all on display are bonus and aids the useability of your site for that user but likewise a mobile device's web browser doesn't afford the physical space to display those links in the same manner.

For the mobile user you may want to use a single drop down menu thats not necsassarily on display the whole time. 

When combined with a responsive design you have the ultimate experience for both traditional desktop users and mobile users are no longer an after thought.