Web application or webapp is an application that is accessed with a Web browser over a network such as the Internet or an intranet. Web applications are popular due to the ubiquity of the 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 many other functions.

In earlier types of client-server computing, each application had its own client program which served as its user

interface and had to be separately installed on each user's personal computer. An upgrade to the server part of the application would typically require an upgrade to the clients installed on each user workstation, adding to the support cost and decreasing productivity.

In contrast, Web applications dynamically generate a series of Web documents in a standard format supported by common browsers such as HTML/XHTML. Client-side scripting in a standard language such as JavaScript is commonly included to add dynamic elements to the user interface. Generally, each individual Web page is delivered to the client as a static document, but the sequence of pages can provide an interactive experience, as user input is returned through Web form elements embedded in the page markup. During the session, the Web browser interprets and displays the pages, and acts as the universal client for any Web application.

The Web interface places very few limits on client functionality. Through Java, JavaScript, Flash and other technologies,

application-specific methods such as drawing on the screen, playing audio, and access to the keyboard and mouse are all possible. General purpose techniques such as drag and drop are also supported by Java, though this may be simpler with current JavaScript libraries. Web developers often use client-side scripting to add functionality, especially to create an interactive experience that does not require page reloading (which many users find disruptive). Recently, technologies have been developed to coordinate client-side scripting with server-side technologies such as PHP. Ajax, a web development technique using a combination of various technologies, is an  example of technology which creates a more interactive experience.

A significant advantage of building Web applications to support standard browser features is that they should perform as specified regardless of the operating system or OS version installed on a given client. Rather than creating clients for MS Windows, Mac OS X, GNU/Linux, and other operating systems, the application can be written once and deployed almost anywhere. However, inconsistent implementations of the HTML, CSS, DOM and other browser specifications can cause problems in web application development and support. Additionally, the ability of users to customize many of the display settings of their browser (such as selecting different font sizes, colors, and typefaces, or disabling scripting support) can interfere with consistent implementation of a Web application.

Another (less common) approach is to use Macromedia Flash or Java applets to provide some or all of the user interface. Since most Web browsers include support for these technologies (usually through plug-ins), Flash- or Java-based applications can be implemented with much of the same ease of deployment. Because they allow the programmer greater control over the interface, they bypass many browser-configuration issues, although incompatibilities between Java or Flash implementations on the client can introduce different complications. Because of their architectural similarities to traditional client-server applications, with a somewhat "thick" client, there is  some dispute over whether to call systems of this sort "Web applications"; an alternative term is "Rich Internet Application".

Though many variations are possible, a Web application is commonly structured as a three-tiered application. In its most common form, a Web browser is the first tier, an engine using some dynamic Web content technology (such as ASP or ASP.NET, CGI, JSP, or PHP) is the middle tier, and a database is the third tier. The Web browser sends requests to the middle tier, which services them by making queries and updates against the database and generating a user interface.

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.

There are many Web application  frameworks which facilitate  rapid application development by  allowing the programmer to define  a high-level description of the  program. In addition, there is  potential for the development of applications on Internet Operating  Systems, although currently there  are not many viable platforms that  fit this model.

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.

The Web Application Security Consortium (WASC), CGI Security, and OWASP are projects developed with the intention of documenting how to avoid security problems in Web applications.

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