[via RossCode]
AJAX (or Asynchronous JavaScript And XML) is used to submit a server request from a client to the server via JavaScript and XML via SOAP. It's best known for being used in the GMail interface.
» WikiPedia: AJAX» Ajax: A New Approach to Web Applications (the original article which coined the term)
Update 5/4: Two great articles about AJAX on CodeProject.com:
» AJAX WAS Here - Part 1: Client Side Framework» AJAX Was Here - Part 2: ASP.NET Integration