Web applications are software programs that run on web servers and are accessed by users through web browsers over the internet or a local network. Unlike traditional desktop applications that are installed on a user's computer, web applications are delivered through web browsers, making them accessible from various devices and platforms, such as desktop computers, laptops, tablets, and smartphones.

Web applications are built using a combination of technologies, including:

1. Client-side technologies:

- HTML (Hypertext Markup Language): Responsible for structuring the content and defining the layout of web pages.

- CSS (Cascading Style Sheets): Used for styling and designing the visual presentation of web pages.

- JavaScript: A programming language that adds interactivity, dynamic behavior, and client-side logic to web applications.

2. Server-side technologies:

- Programming languages: Web applications are typically built using server-side programming languages such as PHP, Python, Ruby, Java, .NET languages (C#, VB.NET), or Node.js (JavaScript).

- Web frameworks: Developers often use web frameworks like Ruby on Rails, Django (Python), Laravel (PHP), ASP.NET (C#), or Express.js (Node.js) to simplify the development process and provide a structured approach to building web applications.

- Databases: Web applications frequently store and retrieve data from databases like MySQL, PostgreSQL, SQLite, or NoSQL databases like MongoDB or Cassandra.

3. Communication protocols:

- HTTP (Hypertext Transfer Protocol): The primary protocol used for communication between web browsers and servers.

- Web services and APIs: Web applications often communicate with other applications or services using web services and APIs (Application Programming Interfaces) that follow protocols like SOAP (Simple Object Access Protocol) or REST (Representational State Transfer).

Web applications offer several advantages over traditional desktop applications, including:

1. Cross-platform compatibility: Web applications can run on any device with a web browser, eliminating the need for separate versions for different operating systems.

2. Accessibility: Users can access web applications from anywhere with an internet connection, without the need to install software on their devices.

3. Automatic updates: Web applications are updated on the server-side, ensuring that all users have access to the latest version without manual updates.

4. Collaboration and data sharing: Web applications facilitate collaboration and data sharing among users in different locations.

Examples of popular web applications include email clients (Gmail, Yahoo Mail), social media platforms (Facebook, Twitter), online office suites (Google Docs, Microsoft Office 365), e-commerce platforms (Amazon, eBay), and many more.

Web applications have become an integral part of modern computing, enabling users to access a wide range of services and functionalities through their web browsers, driving productivity, collaboration, and convenience in various domains.