HTML is the markup language used to produce documents on the Web.
The HTML specification also defines an XML-based way of writing HTML documents called XHTML.
HTML5 was the version of HTML introduced by the W3C as the successor to HTML 4.01.
The name HTML5 has caused confusion since its inception, however, as what was published as HTML5 was in fact modified snapshots of ongoing work being done by a group of browser makers, leaving the web community with two similar standards for the same thing, one versioned and one not.
An agreement has since been reached to stop the dual process of standardizing HTML. As a result, only a single HTML specification is now published without a specific version number.
As HTML5 was the last versioned number to exist, it is still commonly used to refer to the standard, but there is no technical difference between saying HTML5 and HTML anymore. Both refer to the evolving standard.