what is the major difference between xml and html?

XML - A metalanguage written in SGML that allows one to design a markup language, used to allow for the easy interchange of documents on the World Wide Web. HTML - A markup language used to structure text and multimedia documents and to set up hypertext links between documents, used extensively on the World Wide Web.

The best way I can put it with XML is that it is like writing in the English Language. You can create your own custom tags but in order for them to be viewed on a web page, you'll need to know something like XSL or CSS. I don't think XSL works in all browswers but I know it works on IE. CSS I know will work. Hope this makes sense.