Quick definition: It’s the new verson of HTML.
More Reading: HTML5 is the term for the new version of HTML, well what is HTML? HTML is hyper text mark-up language and is the code used to build a web page, all browsers understand HTML and will build a web page out of it. So why change it? This new version of HTML is all about roling with new technologies, as they are developed browser vendors are integrating them into their browsers. What does this mean to me? If you are having a site built then your developer should be fully aware of HTML 5 and all its new features making sure your site can take advantage of these.