Importance of XML, XHTML, HTML5, RAILS, PYLONS

Question

Say, I am creating a web project. I know where I would use HTML, CSS, javascript, and PHP(WAMP).

Now, Where and why would I use XML, XHTML/HTML5(wow it says vector graphics are possible with HTML5?), and Rails or Pylons?

I'm sorry if this looks like a n00b question. I'm not asking how to learn, or what it is - just where and why in a web project would I - if I have to - use it.

Answer 1

You would use XHTML or HTML5 instead of HTML, because XHTML and HTML5 are specific versions of HTML. HTML5 is the newest one.

You would use Ruby or Python instead of PHP, because you prefer one of them over the other ones.

You would use XML when talking to a foreign web service like Twitter, because you need to serialize data in some way. You can also use JSON instead of XML.