Abbreviation for Extensible Hypertext Mark-up Language and is a hybrid of XML and HTML. Web pages designed in XHTML should look the same across all platforms. Basically, XHTML is HTML 4.0 that has been rewritten to comply with XML rules.
Extensible Markup Language, otherwise known as XML. XML is a markup language used for writing custom markup languages. In other words, XML describes how to write new languages (it’s sometimes referred to as a “meta” language because of this). It also serves as a basic syntax that allows different kinds of computers and applications to share information without having to go through multiple conversion layers.
XML Sitemaps are an easy way for webmasters to inform search engines about pages on their sites that are available for crawling. In its simplest form, a Sitemap is an XML file that lists URLs for a site along with additional metadata about each URL (when it was last updated, how often it usually changes, and how important it is, relative to other URLs in the site) so that search engines can more intelligently crawl the site. Web crawlers usually discover pages from links within the site and from other sites. Sitemaps supplement this data to allow crawlers that support Sitemaps to pick up all URLs in the Sitemap and learn about those URLs using the associated metadata. Using the Sitemap protocol does not guarantee that web pages are included in search engines, but provides hints for web crawlers to do a better job of crawling your site. Sitemap 0.90 is offered under the terms of the Attribution-ShareAlike Creative Commons License and has wide adoption, including support from Google, Yahoo!, and Microsoft.