From Mearls latest column on the WotC website...
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.
If Basic D&D is the equivalent of the classic Rules Cyclopedia, then the three core rulebooks are analogous to Advanced Dungeons & Dragons. Want more character options? Pick up a Player’s Handbook. Looking for more critters for your campaign? The Monster Manual has you covered. Want to sculpt a unique campaign? Pick up the Dungeon Master’s Guide. Still, Basic D&D is the true heart of the game and could easily provide a lifetime of gaming.
At the launch of the D&D Starter Set, Basic D&D will include the material needed to create characters and advance to 20th level. In August, with the release of the Player’s Handbook, Basic D&D will expand to include the essential monsters, magic items, and DM rules needed to run the game, along with the rules for wilderness, dungeon, and urban adventuring. (The Starter Set already covers the aspects of these rules that you need to run the included campaign.)
As we introduce new storylines like Tyranny of Dragons, we’ll also make available free PDFs that provide all the rules and stats missing from Basic D&D needed to run the adventures tied into the story. The adventures released as part of Tyranny of Dragons are playable without requiring any of the core rulebooks or the Starter Set. With just the Basic Dungeons & Dragonsrules, you can play D&D for years.
Basic D&D makes it easier than ever for new players and DMs to jump into tabletop RPG play. We’re involved in the greatest gaming hobby ever invented. It’s time to bring that hobby to everyone who wants to take part.
Epic win. A fully playable D&D set as a free download. Beginners can get their hands on the core rules for nothing and enjoy a full fledged campaign experience.