A 'Web Application' or 'Web App' is a program that operates on a remote server and is accessed over the Internet via a browser interface. Unlike traditional desktop applications, web applications don't require software installation on the client's device. They typically employ HTML, CSS, and JavaScript for the front-end, along with various technologies for back-end server operations.

Web applications have gained immense popularity due to their accessibility and compatibility across various devices and operating systems. Their range spans from basic tools like email clients and word processors to more complex systems such as project management software and interactive games.

The structure of a web application usually includes a server to handle client requests, application servers for data processing, and databases for data storage. The client-side logic in the browser interacts with the server-side logic on the application server, which oversees data retrieval, processing, and storage.

Recent advancements in web technology and the advent of cloud computing have greatly enhanced web applications' functionality and performance. Today's web apps offer advanced user experiences that are quick, responsive, and visually compelling, often matching the quality of native desktop or mobile apps.

In summary, web applications mark a pivotal change in software development and usage. They provide benefits like universal access, easy maintenance, and scalability, becoming an integral part of the modern digital world.