Official Comprehensive Rules Site
Since the early days of Magic: The Gathering the rules changed quite a bit. In fact Wizards of the Coast updates the Comprehensive Rules regularly, with the release of most new sets.
The Comprehensive Rules are an exhaustive compilation of all rules used in the game and are available from Wizards of the Coast in TXT, PDF, and DOCX formats. They are about 230 pages long, which is usually a bit much if you just want to look up a keyword, when Instants or Sorceries can be cast or if you are unsure what an Interrupt is.
The Rules Portal gives you the opportunity to browse the Comprehensive Rules in wiki fashion to get exactly the information you want and need. The rules excerpts you find on the pages are from the up to date Comprehensive Rules and the specific pages contain other additional information like examples or clarifications, to give a better understanding how certain rules work in practice.
Keywords and Ability Words