HTML5 is the latest evolution of the standard that defines HTML.
The term represents following key concepts:
Semantics: allowing you to describe more precisely what your content is.
Connectivity: allowing you to communicate with the server in new and innovative ways.
Offline & Storage: allowing webpages to store data on the client-side locally and operate offline more efficiently.
Multimedia: making video and audio first-class citizens in the Open Web.
2D/3D Graphics & Effects: allowing a much more diverse range of presentation options.
Performance & Integration: providing greater speed optimization and better usage of computer hardware.
Device Access: allowing for the usage of various input and output devices.
Styling: letting authors write more sophisticated themes.