If you are a new or returning Diablo fan, you should start to update your knowledge by reading through the Diablo 3 Basics article, which is the main Diablo III Fact Sheet. It provides all the known information about the upcoming Diablo sequel with a brief explanation and links to read more in-depth information.
If you already are a more informed Diablo fan, you are encouraged to help the collective effort of keeping it up to date! You can find out how to help, and what to help with through the help section of the wikis.
Regardless if you just heard that Diablo III is announced, or if you have been following some of the news, the basics page should be able to get you up to date on any given topic or major discussion. It contains information on classes and skills, monsters and items, what NPCs will be featured. The fact sheet even goes into known details about release date, system requirements and age rating.
Basically, everything worth knowing, at a glance, with ample opportunity to delve further into specific topics. Read more in the Diablo 3 Basics.