The "Defiant Map Pack" will feature three additional maps, one of which is a Firefight map, and additional Achievements for an total of 150 Gamerscore will also be included.
You can check out Bungie's official announcement here, and view the trailer in said announcement or below.
I confess, the maps look visually stunning. I really like the backdrop of "Condemned," with Reach being glassed in the background. "Highlands," with its Covenant Cruisers overhead, is also impressive and a nice treat for fans of the expanded universe. Still though, I can't justify paying for Multiplayer Map Packs, so this'll be a pass for me.
Another interesting point is the maps are being developed by Certain Affinity and not Bungie themselves.