While no specifics of the content included has been revealed yet, we do know that it will be multiplayer focused. The blog states that information on the available maps and modes will be shared at a later date, as of now, this is all they are willing to give us:
"During the beta, fans will have an opportunity to try the new momentum-based, chained movement system, find the perfect Specialist to fit their play style, get a feel for the new multiplayer maps, and more."
Access to the beta is limited to PS4, Xbox One and PC and requires a pre-order. Call of Duty: Black Ops 3 is set to release on November 6th this year on PlayStation 4, PlayStation 3, Xbox 360, Xbox One and PC.
Source: Activision Games Blog