In software engineering, a web application is an application delivered to users from a web server over the World Wide Web. 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 clients is another reason they are popular. Applications like webmail, Amazon.com and Ebay are well known examples of web applications but they have uses in many other areas of business and science.
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 created using some dynamic web content technology (e.g., CGI, PHP, Java servlets or Active Server Pages) 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-accessible access to software that has heretofore been distributed as local applications. 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.
