Responsive web design is an approach to web design and development aimed at creating websites that adapt and respond to different devices, screen sizes, and orientations. The goal is to provide an optimal viewing and user experience regardless of whether the website is accessed on a desktop computer, laptop, tablet or smartphone.

Traditionally, websites are designed for specific screen sizes and are usually optimized for desktop computers. However, with the proliferation of mobile devices and different screen sizes, it has become imperative to design websites that adapt to different resolutions and aspect ratios.

Responsive web design achieves this adaptability through a combination of flexible layouts, fluid grids, and media queries. The main principles and techniques used in responsive web design are:

1. Fluid grid:
   Responsive design uses relative units such as percentages and ems instead of using fixed pixel-based measurements for layout elements. This allows the layout to scale proportionally to the screen size, allowing content to scale and flow smoothly.
2. Flexible images and media:
   Images and media elements such as videos and embedded content should also be flexible. It can be styled using CSS properties like max-width.
   100% keeps the width of the container while preserving the aspect ratio.
3. Media inquiries:
   Media Queries is a CSS feature that allows developers to apply different styles based on device or viewport properties. By specifying different CSS rules for different screen sizes or device features (such as orientation), the layout and presentation of the website can be tailored to provide the best experience for each specific context.
4. Breakpoints:
   Breakpoints are specific points in the range of screen sizes where the layout of the website needs to change to accommodate different devices. By defining breakpoints in the CSS using media queries, developers can adapt the design at specific screen widths and rearrange elements, adjust font sizes, or hide/show certain content to improve readability and usability.

By combining these techniques, responsive web design ensures that the website's layout, content, and user interface adjust dynamically to fit the screen size and capabilities of the device being used. This approach allows users to access and interact with the website effectively, regardless of whether they are using a desktop computer with a large screen or a small smartphone display. Given the wide variety of devices and screen sizes used to access the Internet, responsive web design is becoming increasingly important in the modern web environment.