XHTML:

Extensible Hypertext Markup Language (XHTML) evolved from the hypertext markup language (see “HTML”) used for creating web pages. It is based on the HTML 4.0 syntax, but has been modified to follow the guidelines of XML (see “XML” below). Therefore, XHTML 1.0 is sometimes referred to as HTML 5.0.

Because XHTML is extensible, web developers can create their own objects and tags for each web page they build. This gives developers more control over the appearance and organization of their web pages. The only requirement is that the custom tags and attributes are defined in a document type definition (DTD), that is referenced by the XHTML page.

XHTML pages must also conform to a more strict syntax than regular HTML pages. While web browsers are rather lenient and forgiving of HTML syntax, XHTML pages must have perfect syntax. This means no missing quotes or incorrect capitalization in the markup language. While the strict syntax requires more meticulous web page creation, it also ensures web pages will appear more uniform across different browser platforms.

XML:

Extensible Markup Language (XML) is used to define documents with a standard format that can be read by any XML-compatible application. The language can be used with HTML pages, however, XML itself is not a markup language. Instead, it is a “metalanguage” that can be used to create markup languages for specific applications. For example, it can describe items that may be accessed when a web page loads and allows you to create a database of information without having an actual database. While it is commonly used in web applications, many other programs can use XML documents as well.