It's a prequel to The Passing DLC which had 1 & 2's survivors meeting up. Sacrifice for L4D2 gives you both Sacrifice and L4D's No Mercy campaign.
Left 4 Dead will receive The Sacrifice campaign with maps playable in Campaign, Versus and Survival. L4D2 has The Sacrifice and No Mercy playable in Campaign, Versus, Survival and Scavenge.
L4D2 also includes a Special Infected, items and weapons. The Sacrifice releases October 5th on Xbox 360 for 560 MSP, while the PC enjoys the new content for free.