The web is an ever-changing medium with all the advancement in the technology. There is no limit to the number of devices on which a website can be viewed. The screen size, resolution, audience, support etc changes with each device and browser on which you are accessing a website.
The worst part of accessing a website over different devices (of different screen size, resolution etc) is that you want to see the relevant information in a user-friendly manner, while what you see is a distorted layout with all the web components spread unevenly all over the screen. The designers used to create websites to be viewable nicely on different browsers on a computer screen, now need to work on how the website will look on different phones, tablets, netbooks etc.
The solution which designers used to suggest sometime back was to create a separate version of the website which is specifically for the mobile devices & the tablets. But is this really what we want? What if there is another device in the market which is neither of these?
No one wants different websites for different devices!!
This problem was resolved with the introduction of adaptive web designs.Adaptive web designs allows a user to view their websites on different devices, resolution etc and the content will automatically best fit to render the site in best possible way. This is really useful for different screen sizes where displaying less important content on top may even result in a higher bounce rate.
Think of it that if your website is a two column layout website with left column as the navigation and the right column displaying some useful information. If your website is viewed on a mobile device and the links are displayed on the top rather than the content, the user looking for relevant content might be drifted away, while if the content is on top, then it might interest the user.
Adaptive design is not just limited to this. Adaptive designs also focuses on displaying the components which are relevant on different devices. For e.g. displaying a print button on a mobile device might not make much sense as compared to displaying a print button on the desktop website.