AJAX stands for asynchronous Javascript and XML, and is one of the key visible building box in popular Web 2.0 technologies. Ajax is a mixture of several technologies that integrate Web page presentation, interactive data exchange between client and server, client side scripts, and asynchronous updates of server response. The Ajax intermediary sits on the client side sending requests to a server and updating the page asynchronously. A key component of the open standards–based AJAX is the Application Programmer Interface called XMLHttpRequest (XHR) that scripting languages use to exchange data between a client and a Web server. The data is often in XML format but can be HTML, text, Javascript arrays, or even a few customized formats. Likewise, the scripting language does not have to be Javascript. XHR is not a protocol extension and was not introduced in any formal manner, rather, it was a feature of Microsoft’s ActiveX extended to other platforms.

The key purpose of Ajax is to let scripts act as HTTP (or HTTPS) clients and send/receive data from Web servers using a variety of common HTTP methods (GET, HEAD, POST, PUT, DELETE, and OPTIONS are supported currently). Thus, Ajax can be used for dynamic layout and reformatting of a Web page, reduce the amount of reloading needed by sending a request for just a small portion, and interact on demand with the server. The responses from the server are handled asynchronously by the browser without having to keep the user’s attention frozen. Numerous popular dynamic Web applications, such as maps, use XHR.

Similarly, Flash objects can offer similar functionality in that once downloaded they can communicate asynchronously with a server. Consequently, YouTube videos can begin playing before the whole movie has been received: the user downloads a compact flash object which downloads a small prefix of the video and begins playing it out while asynchronously fetching the remainder of the video. Supporting Flash requires an appropriate Adobe plug–in to be installed, although user penetration of this plug–in is in the high ninety percentiles. Toolkits exist allowing Internet applications to be written in a high level language and then rendered either as Flash objects or pages with Ajax components, meaning that it may be helpful to sometimes think of Flash and Ajax merely as object code. Ajax apps are typically easier for the researcher to reverse–engineer and understand for measurement purposes than the Shockwave Flash (SWF) format. Currently, Flash is mostly used for rendering rich embedded objects (video, audio, games): few entire applications which store and recall data are implemented in Flash.
https://firstmonday.org/article/view/2125/1972#p4