Responsive design is website design that adapts to viewer’s device and viewable area. Mobile internet usage trend is growing fast. If you look at your website stats then you will notice grows of mobile visitors in percentage. Especially if it is classifieds site then users always search for things to buy, rent, sell. Responsive design makes it possible to deliver same content if you visit site from desktop computer or mobile phone or tablet. Every element on web page will restyled according to mobile device by hiding some unnecessary fields, resizing images and repositioning some elements.
Update 07.01.2014: Google added responsive ad units you can view more info about it here.
Amit Agarwal provided a google approved solution for responsive websites using adsense ads. In our case we want to Leaderboard (728 x 90) ad to be replaced with Mobile Leaderboard (320x50) ad. This solution will work on page load which is how it should work for mobile device, ad will not change if you are resizing page after it is loaded on your desktop computer.
Please keep in mind that you can use only one mobile leaderboard ad per page.
For all mobile ad units with the exception of the mobile leaderboard, one or more text ads are eligible to show.
In same manner you can replace wide link units with compact ones if you are using google adsense link units.