Traditionally web applications were meant to serve static content, wherein the server generated an HTML response to the client's request (typically HTTP) and sent it back to the client. The response was then rendered on user's screen (browser window) by the client (browser). In order to increase user-perceived performance, such static content was cached at the edge of Internet so that it could be served faster. Content Delivery Networks (CDNs) have been used commercially for such purposes.
As the web-application ecosystem evolved, rich-internet applications and mashups got popular. Rich-Internet applications meant that data (a web page) served by the server is dynamic i.e. it is a function of the state of client's request (logged in user, cookies, session variables etc).