A web application or "web app" is a software program that runs on a web server. Unlike traditional desktop applications, which are launched by your operating system, web apps must be accessed through a web browser.
Web apps have several advantages over desktop applications. Since they run inside web browsers, developers do not need to develop web apps for multiple platforms. For example, a single application that runs in Chrome will work on both Windows and OS X. Developers do not need to distribute software updates to users when the web app is updated. By updating the application on the server, all users have access to the updated version.
From a user standpoint, a web app may provide a more consistent user interface across multiple platforms because the appearance is dependent on the browser rather than the operating system. Additionally, the data you enter into a web app is processed and saved remotely. This allows you to access the same data from multiple devices, rather than transferring files between computer systems.
Web design is the process of creating websites. It encompasses several different aspects, including webpage layout, content production, and graphic design. While the terms web design and web development are often used interchangeably, web design is technically a subset of the broader category of web development.