HTML is a markup language with defined tags
XML is a markup language where you make up your own tags
XHTML is an XML implementation of HTML
XML is a markup language for documents containing structured information.
Structured information contains both content (words, pictures, etc.) and some indication of what role that content plays (for example, content in a section heading has a different meaning from content in a footnote, which means something different than content in a figure caption or content in a database table, etc.). Almost all documents have some structure.
A markup language is a mechanism to identify structures in a document. The XML specification defines a standard way to add markup to documents.
What's a Document?
The number of applications currently being developed that are based on, or make use of, XML documents is truly amazing (particularly when you consider that XML is not yet a year old)! For our purposes, the word "document" refers not only to traditional documents, like this one, but also to the miriad of other XML "data formats". These include vector graphics, e-commerce transactions, mathematical equations, object meta-data, server APIs, and a thousand other kinds of structured information.
So XML is Just Like HTML?
No. In HTML, both the tag semantics and the tag set are fixed. An <h1> is always a first level heading and the tag <ati.product.code> is meaningless. The W3C, in conjunction with browser vendors and the WWW community, is constantly working to extend the definition of HTML to allow new tags to keep pace with changing technology and to bring variations in presentation (stylesheets) to the Web. However, these changes are always rigidly confined by what the browser vendors have implemented and by the fact that backward compatibility is paramount. And for people who want to disseminate information widely, features supported by only the latest releases of Netscape and Internet Explorer are not useful.
HTML is well known Hypertext Markup Language that provide the good set of markup tags which are used to create a static web pages while XML is an Extensible Markup Language that allows users to create own tag. HTML is designed to display data where XML is designed to describe the data. HTML is not Case Sensitive where XML is Case Sensitive.