HTML5 is the next major revision of the HTML spec. The following four elements / attributes can be considered the most important additions to HTML5.
Graphics, visuals, or animations can be rendered in real time without relying on plug-ins. Rich interactive sites or games can now be created without Flash. This opens up the possibility of having really complex experiences and interactions in a light footprint.
Geolocation can determine the user’s physical location and share with trusted web sites. Users will need to opt-in before this information can be shared. It is more accurate than current methods. We can now display relevant local information (weather, restaurants, businesses, etc…) to the user. Or even track a user’s progress on a cross country drive.
Storage enables web pages to store data within the client’s browser. What does this mean? For example if the user is in the process of searching for a product, filling out a really long form, or in the middle of a TV show on Hulu and closes the browser window by accident, the user would lose their progress. But with HTML5 Storage, progress can be saved locally, and we can offer the user the ability to go back to where they were.When can we start using HTML5?
See which browsers support them now: http://html5readiness.com