HTML is the most common language used today to develop web sites. Although for the most part, it is sufficient enough for most web developers and will display correctly in the most popular browsers, HTML does have its share of drawbacks. XHTML (Extensible Hyper Text Markup Language) was developed to correct everything that is wrong with HTML. XHTML is set out to replace HTML and be used as the default language for developing web sites.
As the internet expands, people are able to access web sites from handheld devices such as PDA's and cell phones. These devices do not have the resources to render documents using bad HTML. Therefore XHTML is a stricter and cleaner version of HTML which allows web browsers to more easily render web pages since they must follow specific rules. XHTML is almost identical to HTML 4.01 as it uses the same tags and attributes but many syntax rules apply to XHTML documents which you do not have to worry about when writing HTML code.
XHTML was designed to be backwards compatible so that you can begin to write all your web sites in XHTML and have them work in both new devices as well as older web browsers.