HTML5 is the standard that was never supposed to be needed. W3C promoted XML as the new Web standard but, understandably, it failed to take off. Why? Too complicated to implement and, hence, too expensive for the majority of organizations with a Web presence - not to mention individuals with home pages. So, the folks at Opera got busy with preparing a draft version of HTMl5, which was eventually picked up by others and finally accepted by W3C as inevitable.
There's general agreement that HTML5 has an excellent set of features, including the ability to play video without needing third-party software like Flash, and Canvas, its drawing feature. However, the new tags are also thought to be very valuable in enabling a greater definition of page layout. Some of the new tags have been referred to as 'semantic' tags, but I'm afraid this isn't the case. There is no such thing as a semantic tag in HTML5.
The tags that are said to be "semantic" include <header>, <footer>, <section>, <nav>, <article> and one or two more - some of which are not yet formally accepted (indeed, HTML5 as a whole is not yet formally accepted as a new standard). Let's look at what "semantic" means; the OED is quite brief in its definitions: "Relating to signification or meaning" and "Also (the study or analysis of) the relationships between linguistic symbols and their meanings".