Sajax is an open source tool to make programming websites using the Ajax framework — also known as XMLHTTPRequest or remote scripting — as easy as possible. Sajax makes it easy to call PHP, Perl or Python functions from your webpages via JavaScript without performing a browser refresh. The toolkit does 99% of the work for you so you have no excuse to not use it.
jQuery is a new type of Javascript library. It is not a huge, bloated framework promising the best in AJAX - nor is it just a set of needlessly complex enhancements - jQuery is designed to change the way that you write Javascript. What is jQuery?