In DCUO, things are broken down a little differently than some other MMOs - rather than the DPS/healer/tank trinity, you've got controller/healer/tank, and everyone does DPS.
The game's name for "tank" is "Defense role". There are two flavors of Defense role - Fire and Ice.
Fire's official description:
"Defense: Command the power of flame, directing gusts of heat and protecting allies with targeted blasts."
Ice's official description:
"Defense: Command the power of ice, shaping it into mighty weapons and protecting allies with frigid winds."
While neither of those sounds like "tanking", a peek at the powers in the Ice tree tells us more:
There are two signature sub-trees within Ice - Storms and Cryogenics. The very first Ice->Storms power is "Inescapable Storm", which "Engulfs an enemy and others nearby, dragging them toward you. Inflicts Frostbite to make enemies vulnerable to freezing effects."
That sounds pretty tanky - pull enemies to you and put them in a state where your more advanced attacks will control them in a variety of ways. (Example: Later power "Freeze Wave", which has a chance to root or freeze enemies affected by Frostbite.)
Meanwhile, Ice->Cryogenics' first power is "Cold Snap", which "Plummets your body temperature, increasing your resistance to control effects, damaging nearby enemies, and reflecting additional damage back at your attackers when they strike you. Grants Chill Effect; gaining 5 consecutive Chill Effects activates Ice Armor."
Again, pretty tanky, albeit more defensively-oriented.
So that's how the DCUO tank works on paper. I haven't played far enough in to say how the tanking experience compares to other games.
The game itself and the online game guide.