Javascript and Ajax

What is the difference between javascript and ajax? And which one is best to use?

AJAX is Javascript, so there's no difference in languages.

AJAX uses Javascript to interact asynchronously with the server. That's the only "difference" to what most people think of as standard Javascript.

As for which one to use, that will depend completely on what you want to use it for. Different tools for different purposes.

1. Javascript-A language that comes with your browser. It allows you to show an alert box, the one that you need to press Ok, validate forms, on the client, or without change the page. Usually, it is stopped by an alert, confirm, etc box.
2. AJAX-Asynchronous Javascript And XML. Which means that with a help from a programming language like PHP, ASP.NET and etc. it can work changing data without refresh the whole page, just the part of the data will be refreshed.