A web application is an application that uses a web browser as its primary client. It is coded using browser-supported languages like HTML, PHP, JavaScript, and runs on the client-side. Some key advantages are that they do not require installation, can be accessed from anywhere, and integrate easily with server-side apps. However, they also rely on an internet connection and the hosting server. The history of web apps began in 1995 with JavaScript, and technologies like Flash, Ajax, and HTML5 have continued advancing their capabilities since.