Extensible Hypertext Markup Language, or commonly known as XHTML is the big brother of HTML. The changes between XHTML 1.0 and HTML are very minor and are there to mainly conformance with XML standards. The main and most important change is a document must be well formatted and all elements and tags are closed as required in XML. This means you must define all tags in lowercase.
All elements must also be closed, including any empty elements such as img, br, and meta. Closing these elements is as simple as adding a closing slash to the end of the tag.
If you're looking for more info regarding what XHTML is and why it's around, feel free to check out the W3C XHTML 1.0 specifications for complete documentation.
To get a better feel and understanding of how to program in XHTML, it's recommended you have a basic understanding of HTML.