different resolutions
Until recently a 1024 x 768 pixels resolution was the standard on the internet. Since the rise of the iPhone, iPad, Android and tablets many different resolutions are used.
Most sites are not designed for smaller, rotating screens and touch screen control. This requires a different approach. Responsive web design is a relatively new method to operate on all modern platforms.
tablet, smartphone
Developers used to build a separate version of the site for mobile phones. The disadvantage is that if you sent someone this mobile link he also sees the mobile version when he looks on his computer.
Responsive web design is based on HTML5 and CSS3. The site automatically adapts to the device and the resolution of the visitor. You always see the same content and website but the display is different.