In computer software engineering, a web application is an application coded in a browser-supported language, such as HTML, Java or JavaScript. The application is executed via common web browsers over a network such as the Internet or an intranet. Popular web applications include webmail, online retail sales, discussion boards, weblogs, multiplayer online games, etc. Their popularity derives from the fact that they can be accessed online, saving the trouble of installing and updating software applications on potentially hundreds of thousands of individual user computers.

A web application can be considered as a two-tier construction. The web browser, where everything is presented to the user via HTML pages, is the first tier. The second tier is an engine that uses certain dynamic web content technology, such as JSP, ASP or PHP, to serve the user by making queries and updates against a database. When translating web applications for the purpose of internationalization or localization, translators only need to focus on the interface, i.e. the first tier. Elements to be translated include onscreen messages, error messages, labels, tab labels, button tops, dialogs, pick lists, drop-down lists and hyperlinks, just to name a few. These should be translated clearly, precisely and free of error.