Difference between html/Xhtml

XHTML is a combination of HTML and XML (Extensible Markup Language).
XML is a markup language where everything must be marked up correctly,
which results in "well-formed" documents. XML is designed to describe data,
and HTML is designed to display data. Today's market consists of different browser technologies, some browsers run on computers, and some browsers
run on mobile phones or other small devices. The last-mentioned do not have
the resources or power to interpret a "bad" markup language. Therefore - by combining the strengths of HTML and XML, W3C recommended a markup language that is useful now and in the future - XHTML.

The Most Important Differences:

1.XHTML elements must be properly nested

This text is bold and italic
2.XHTML elements must always be closed

This is a paragraph
This is another paragraph
3.XHTML elements must be in lowercase

<body>
This is a paragraph
</body>
4.XHTML documents must have one root element.
All XHTML elements must be nested within the <html> root element.
Child elements must be in pairs and correctly nested within their parent element.
The basic document structure is:

<html>
<head> ... </head>
<body> ... </body>
</html>

references

http://www.w3schools.com/tags/tag_DOCTYPE.asp
http://www.w3schools.com/xhtml/xhtml_syntax.asp