HistoryIn the 1990s, most web sites were based on complete HTML pages; each user action required that the page be re-loaded from the server (or a new page loaded). This process is inefficient, as reflected by the user experience: all page content disappears then reappears, etc. Each time a page is reloaded due to a partial change, all of the content must be re-sent instead of only the changed information. This can place additional load on the server and use excessive bandwidth.
The term Ajax was coined on February 18, 2005 by Jesse James Garrett in an article entitled Ajax: A New Approach to Web Applications.
On April 5, 2006 the World Wide Web Consortium (W3C) released the first draft specification for the XMLHttpRequest object in an attempt to create an official web standard.
TechnologiesThe term Ajax has come to represent a broad group of web technologies that can be used to implement a web application that communicates with a server in the background, without interfering with the current state of the page. In the article that coined the term Ajax, Jesse James Garrett explained that the following technologies are incorporated:
- HTML or XHTML and CSS for presentation
- the Document Object Model (DOM) for dynamic display of and interaction with data
- XML for the interchange of data, and XSLT for its manipulation
- the XMLHttpRequest object for asynchronous communication
- Pages dynamically created using successive Ajax requests do not automatically register themselves with the browser’s history engine, so clicking the browser’s “back” button may not return the browser to an earlier state of the Ajax-enabled page, but may instead return to the last full page visited before it. Workarounds include the use of invisible iframes to trigger changes in the browser’s history and changing the URL fragment identifier (the part of a URL after the ‘#’) when Ajax is run and monitoring it for changes.
- Dynamic web page updates also make it difficult to bookmark a particular state of the application. Solutions to this problem exist, many of which use the URL fragment identifier (the part of a URL after the ‘#’) to keep track of, and allow returning to, the application in a given state.
- Depending on the nature of the Ajax application, dynamic page updates may interfere disruptively with user interactions, especially if working on an unstable internet connection. For instance, editing a search field may trigger a query to the server for search completions, but the user may not know that a search completion popup is forthcoming, and if the internet connection is slow, the popup list may show up at an inconvenient time, when the user has already proceeded to do something else.
- The same origin policy prevents some Ajax techniques from being used across domains, although the W3C has a draft of the XMLHttpRequest object that would enable this functionality. Methods exist to sidestep this security feature by using a special Cross Domain Communications channel embedded as an iframe within a page, or by the use of JSONP.
- Ajax-powered interfaces may dramatically increase the number of user-generated requests to web servers and their back-ends (databases, or other). This can lead to longer response times and/or additional hardware needs.
- The asynchronous, callback-style of programming required can lead to complex code that is hard to maintain or debug. 
- Ajax cannot easily be read by screen-reading technologies, such as JAWS, without hints built into the corresponding HTML based on WAI-ARIA standards. Screen-reading technologies are used by individuals with an impairment that hinders or prevents the ability to read the content on a screen.