It may be a bit 2012 but it’s still relevant – responsive web design. What is it? Well, seeing for yourself is the best way: just grab the corner of your web browser and resize the window. Can you see the change? The website is built using a grid system and adjusts the page style to fit the corresponding width (as set by the browser window). If you are reading on a phone or tablet then turn your screen to portrait or landscape to see the same responsive behaviour. Clever eh.
We are big advocates and build our WordPress sites using this method. Here is a fantastic infographic which outlines responsive design.