What is AJAX?
Is it some sort of new technology or language?

Its a mixture of languages, and XML related

AJAX or Asynchronous JavaScript and XML is a term describing a web development technique for creating interactive web applications using a combination of: * HTML (or XHTML) and Cascading Style Sheets for presenting information* Document Object Model, JavaScript to dynamically display and interact with the information presented* XML, XSLT and the XMLHttpRequest object to interchange and manipulate data asynchronously with the web server (although AJAX applications can use other technologies)

examples of ajax use are

digg.com
google suggest
google spreadsheets
infact a lot of google things
also vBulliten uses AJAX for quick replys and edits

why we need of AJAX is PHP is not enough for doing this job.Another question is,can we user ASP instead PHP i mean is it the alternative to PHP?