Wizards of the Coast announced plans to release Basic D&D as a free PDF downloadable from its website in July, coinciding with the release of the D&D Starter Set.
In an article on their website, Mike Mearls, senior manager for the Dungeons & Dragons research and design team, outlined the material that would be included in the Basic D&D download, comparing it to the D&D Rules Cyclopedia of previous editions, but without as much detail.  "We want to put D&D in as many hands as possible, and a free, digital file is the best way to do that."

The release will cover the basic core of the game, including material needed to create characters and advance to 20th level.  It will cover basic classes (cleric, fighter, rogue, wizard) and essential subclasses, as well as basic race options (dwarf, elf, halfling, human).  The introduction is positioned to steer players toward purchasing more extensive material in the Player’s Handbook, Monster Manual, and Dungeon Master’s Guide.  But, Mearls also states, “Basic D&D is the true heart of the game and could easily provide a lifetime of gaming.”
When the Player’s Handbook releases in August, Basic D&D will expand to include essential monsters, magic items and DM rules to run the game, as well as rules for different settings (wilderness, dungeon, urban).  When storylines like Tyranny of Dragons launch, Wizards will make new PFFs available that provide the rules and stats to run the adventures tied to the story.  This way, Mearls says, players are not required to purchase any of the core rulebooks or the Starter Set to participate in adventures.
The free distribution of rules is another example of WotC’s strategy of free sampling to get people involved in the new Dungeons and Dragons; the company is also making the first part of the adventures in its D&D Encounters organized play storyline events free (see Exclusive Interview: "'D&D's' Liz Schuh on the New Edition OP").
For more information on the upcoming D&D releases and organized play, see "New 'D&D' Dates, Info, and Pics" and "'D&D' Adventurers League Launches with New Edition."