Welcome to the Forum!
It can be confusing at first. But I'll give a very basic overview and then point you to where you can get some good info.
XHTML is just HTML with a stricter adherence to some rules. For example - you are required to have proper closing tags on ALL of your elements, and attribute names need to be lower case. As long as you follow the rules you can name it .html and no one is the wiser. The .xhtml is/was just a way to tell the difference for software programs that cared.
CSS2 is the list of attributes that you can use to describe html elements. CSS3 is the new list with extended capabilities/attributes. Depending on the particular reader App/Device they can support any, none, or all of the CSS3 list. If you want the widest possible support of your epub, you should use ePub2 and stick to the CSS2 list.
HTML5 is just the next generation of HTML. It was 4.01, but they added some new capabilities/tags.
ePub2 is what almost all ebooks are written in now - because the reader Apps/Devices don't yet support the advanced functionality in the new version ePub3. This is changing - slowly. Kindle, iBooks, Kobo, are among the few who are STARTING to support the functionality - but they are using their own hybrid...so that's a mess.
The ncx file is what ePub 2 uses to navigate through the documents - think Table of Contents. ePub 3 uses more of an HTML based document for it's navigation.
XML - don't know...I avoid it!
XSLT - don't know about that either, but from the context I would gather it is an interpretation program from one format to another??
OK...pretty quick and dirty.
The W3Schools website has some great tutorials that can explain all of this much better, and they have reference pages as well to show you what all the different tags mean and how they are used.
http://www.w3schools.com/html/default.asp
I hope that helps!
Cheers,