The core game mechanics are identical to Dark Heresy and Rogue Trader. That is, it's a percentile system, and your goal is to roll under your associated characteristic. The Kill-Team (your group of PCs) are much much better armed and armored than any group of Acolytes or Explorers, though. The Battle-Brothers are all capable of wrecking shit, but, based on their home Chapter, they get slightly different abilities and gear that reflect their specialized role. (The assault marine has a powerfist, the devastator marine has a heavy bolter, and so on.)
That said, a single enemy with a lasrifle is no threat to a Space Marine, much less four of them, so Deathwatch introduces rules for hordes of enemies, who can do awful things to you, like overpowering you, dragging you to the ground, and tearing you limb from limb.
Just like Dark Heresy and Rogue Trader, the PCs can spend fate points to turn the tides in their favor. In Deathwatch, spending a fate point will allow you to reroll a failed test, gain a bonus to an upcoming test, add an extra degree of success to a successful test, roll and automatic 10 for initiative, or even heal yourself.
In addition, each Battle-Brother can, once per session, trigger either his Chapter Demeanor or Personal Demeanor to accomplish something really awesome. At the least, a triggered Demeanor provides similar benefits to fate points, but the benefits could be much better if roleplayed appropriately.
