Mojang and Telltale are teaming up to bring the world a "narrative-driven video game." The plan is to make Minecraft: Story Mode as "Minecrafty" as possible but with the obvious Telltale touches.
• Minecraft: Story mode will be a narrative-driven video game created by Telltale Games. It will be about Minecraft.
• Mojang is collaborating with Telltale and members of the community to make Minecraft: Story Mode as Minecrafty as possible.
• The first episode will be released some time in 2015, and will be available on Xbox consoles, PlayStation consoles, PC, Mac, iOS, and Android-based devices.
• Minecraft: Story Mode will be released episodically, just like The Wolf Among Us, The Walking Dead, or the recent Tales from the Borderlands, which is another developer-collaboration series between Telltale Games and Gearbox Software.
• We’re not intending on creating an “official” story for Steve, or explaining the world of Minecraft in detail. It will be a cool game.

It does seem like it'll be a standalone game from normal Minecraft, though obviously details are scarce.