From Bethesda Game Studios:
Fallout Shelter places you in the role of a Vault Overseer in the world of Fallout. You can build your Vault how you want, and assign your Dwellers various duties inside the community. You'll also need to protect them from hazards coming from both inside and outside the Vault.
While you're waiting, head over to iMore for the best strategies for keeping the wasteland out of your vault, so you're ready to go on August 13.
Source: Bethesda Game Studios (Twitter)