Can anyone tell me what exactly AJAX is mostly used for, and how should I start learning how to use this technique? I'm still only beginning web design with HTML/CSS/Javascript, but I've been curious about this for a while and figured I could get a heads up from you guys. Wikipedia was pretty vague, and google just swarmed me with shitty paid online courses.

Check the references in the wikipedia too.
http://www.xul.fr/en-xml-ajax.html
Try that page.

I recommend you learn HTML and CSS first. Javascript is actual programming, which is a major step after HTML. When you're done learning javascript you might be able to follow the rest (or even get yourself involved with computer science/application development instead of web design... if you're interested).

I recommend not using XMLHttpRequest directly. Libraries for AJAX were created for a reason. E.g. with jQuery it can be:

$.post("http://example.com/bar", {"name" : "John Doe"});

That POSTs a hash containing a name to example.com/bar. You're not going to get much easier than that.

Also, it takes two to tango: you can't use AJAX unless there's a piece of code on the server waiting for your specific request, so you'll need to write that too.