Responsive Design
Responsive Design

Definition
Responsive design allows you to design a single website for all devices. The site recognises the type of device and automatically serves a page that responds to the size and capabilities of the device. This means the site also appears to be optimized for any screen size.
A mobile response site just adjusts to the screen size. All the content from the desktop site still needs to be downloaded. This is one reason why a mobile first approach is advisable with a responsive design. The danger with a responsive site is that some of the content will not be relevant to customers on mobile devices. Load speed will also suffer.
Example
Here a desktop site for Salford Van Hire uses a responsive design to create a mobile experience. Unfortunately there is far too much content for a mobile user. The navigation has also not been adapted for the mobile experience.

Image Source: Salfordvanhire.com
Finally, removing too much content from a site may compromise the desktop experience. An adaptive design may also be better for the end user. With the advent of mobile only users it is important to select an option that delivers comprehensive functionality for mobile devices.
Resources:
Conversion marketing – Glossary of Conversion Marketing.
Over 300 tools reviewed – Digital Marketing Toolbox.
A/B testing software – Which A/B testing tools should you choose?
Types of A/B tests – How to optimise your website’s performance using A/B testing.