There was a time when most web surfing occurred on a computer with a standard-sized/ratio monitor. It was more than adequate to create websites with a non responsive layout in mind. But over the last 10 years there has been an exponential boom of new devices in a plethora of form factors, from mobile phones, tablets, watches and a wide range of screen sizes. This growth has created a huge fragmentation problem, so creating websites with a single layout is no longer acceptable. A website with a lot of content that works great on desktops doesn't work very well on a mobile device that has a significantly smaller screen. Such content is unreadable, forcing the user to zoom in and out constantly. One might try making everything bigger so it looks good on mobiles, but then on a desktop, the content doesn't take advantage of the immense real estate offered by bigger screens.
Responsive Web Design is a method that allows the design to respond based on the user's input and environment, and thus based on the size of the screen, the device, and its orientation. This philosophy blends elements of flexible grids and layouts, images, and media queries in CSS.
Enter Responsive Web Design. This alleviates this problem by allowing developers and designers to create websites that adapt to all screen sizes/ratios. There are various approaches that different websites take, but the core concept is illustrated in the following figure:
The preceding figure shows how the same website's layout can be adapted to suit different devices. On the desktop there is a lot more real estate, so the content is bigger and more can fit on a single row. But, as the screen size shrinks and its orientation changes, the content readjusts itself to accommodate this change. This provides a seamless and elegant experience for the user on all form factors. If you look closely at the preceding figure and at modern websites, you will see a grid that the content conforms to. The grid is used to lay out the content of a website, and both of these elements go hand in hand. This grid system is one of the most important aspects of how Responsive Web Design works, and this will be covered in depth very soon.