Mike Mearls, writing on the Dungeons & Dragons blog:
Basic D&D is a PDF that covers the core of the game. It’s the equivalent of the old D&D Rules Cyclopedia, though it doesn’t have quite the same scope (for example, it won’t go into detail on a setting). It runs from levels 1 to 20 and covers the cleric, fighter, rogue, and wizard, presenting what we view as the essential subclass for each. It also provides the dwarf, elf, halfling, and human as race options.
But the best part? Basic D&D is a free PDF. Anyone can download it from our website. We want to put D&D in as many hands as possible, and a free, digital file is the best way to do that.
Very cool that they're doing this. Great way to spread the word around to the tech savvy generation that may have up until now disregarded D&D as a relic of the past. Would love to see how many people this hooks and encourages to get into pen and paper RPGs.