HTML stands for HyperText Markup Language.
HTML4 is the oldest version of HTML that is still used today. It has a loose syntax where things like closing tags are optional. One does not have to close a paragraph with a tag in HTML4 either. The loose syntax helps developers easily adapt to HTML. While loose syntax helps amateur developers, it also poses a problem when one’s trying to achieve a specific result in the browser.
XHTML was introduced to deal with this specific problem. Majority of sites that are developed today use XHTML. XHTML stands for EXtensible Hypertext Markup Language. It is based on XML, a more generalized markup language. In XHTML, all opened tags have to be closed. There are also some restrictions on which tags can be nested inside each other.
HTML5 is the newer version of HTML. It is becoming increasingly popular every day. HTML5 comes with new functionalities that help in how we use the web today. Developers are able to do a lot of exciting new things with the help of HTML5.