HTML5 is the 5th revision of the HTML language created in 1990 for structuring and presenting content on the Internet, and an attempt to improve support for modern multimedia elements.
The fifth revision of HTML attempts to improve the language with support for the modern multimedia elements while still being easily readable by humans. The most "visible" additions are the following syntactical features:
<canvas> which are to replace the
<object> tag (helped by the integration of SVG) and
<nav> which are to semantically enhance the content of documents.
HTML5 is the successor of HTML4, but also of XHTML1 (XHTML2 never went beyond draft). As such, it may be serialized into text/html but also to XML, and this XML serialization has been named XHTML5.