Javascript is a client-side language that runs in the browser.

AJAX Is javascript that for the most part uses the XMLHTTPREQUEST object to grab snippets from a server.

For example.. I have a website. I want some data. I will use AJAX to grab some data from a database. Then I plug that data into the site using Javascript. The user never has to reload the page. It is all real-time. This site is AJAX heavy.

AJAX is a programming technique and a way of designing websites so that bits of a page can be updated without updating the whole page. Javascript is the language that those programming techniques are most commonly used in.

JavaScript is a language. Ajax is a fairly simple IE-normalizing function written with that language used to talk to servers, a bullet-point that will hopefully one day be pointless on your resume, and a buzzword spoken in hushed tones of great power and mystery by people who know very little about JavaScript.