In software engineering, a Web application is an application delivered to users from a Web server over a network such as the Internet or an intranet. Web applications are popular due to the ubiquity of the Web browser as a client, sometimes called a thin client. The ability to update and maintain Web applications without distributing and installing software on potentially thousands of client computers is a key reason for their popularity. Web applications are used to implement webmail, online retail sales, online auctions, wikis, discussion boards, weblogs, MMORPGs, and perform many other functions.

Application: -

Web interfaces have increasingly been used for applications that have previously been thought of as traditional, single-user applications. For example, Microsoft HTML Help replaced Windows Help as the primary help system in Microsoft Windows. Like their networked brethren, such applications generate Web pages as their user interface and send them (sometimes via an embedded HTTP server) to a local Web browser component, which then renders the pages for the user and returns user input to the application. Web applications powered by embedded Web servers have also become commonplace as the user interfaces for configuring network components such as servers, routers, and gateways .

Business use:

An emerging strategy for application software companies is to provide Web access to software previously distributed as local applications. Depending on the type of application, it may require the development of an entirely different browser-based interface, or merely adapting an existing application to use different presentation technology. These programs allow the user to pay a monthly or yearly fee for use of a software application without having to install it on a local hard drive. A company which follows this strategy is known as an application service provider (ASP), and ASPs are currently receiving much attention in the software industry.

Writing Web applications

While many Web applications are written directly in PHP or mod_perl, there are many Web application frameworks which automate the process, by allowing the programmer to define a higher-level description of the program.
The use of Web application frameworks can often reduce the number of errors in a program, both by making the code more simple, and by allowing one team to concentrate just on the framework. In applications which are exposed to constant hacking attempts on the Internet, security-related problems caused by errors in the program are a big issue.

A web application framework is a set of software tools to make it easier to create web applications. They typically provide functionality such as database access, templating and session management.
Such tools range from simple libraries such as DBI and the much larger PEAR, or pure Ajax presentation mechanism such as ZK, to systems using Model-view-controller concepts, where the mere structure of the data is all that need to be specified to create a fully-functional application.

For example, when programming a to-do list as a web application, it is often sufficient for a programmer to specify "each user can have many tasks, each of which has a description, a date, and a priority" and the web application framework will turn that description into a program capable of adding, editing, listing, and deleting tasks from a database