A tank is a creature with usually high hit points (HP) whose primary role is to absorb damage and prevent others from being attacked.
In Dungeon Overlord, specific creatures are given the tank role. These creatures have extremely high HP compared to other creatures, and they are often the slowest creatures in the game. Tank creatures are the only non-endgame creatures that gain Pillage at a reasonable rate, making them necessary for damaging most of the rooms in an enemy Dungeon.
In the Creatures article there is a stats comparison table; two of the rightmost columns are "Tank" and "Tank Range." Creatures with larger numbers in both are either tank creatures or extremely powerful endgame creatures (such as Demons and Dragons, although the former may qualify as tanks). The "Tank" value most likely directly influences the chance that an enemy will target that creature. The "Tank Range" value gives a maximum range at which the tank can influence targeting. This value is extremely important for ogres during Dungeon defense, because the Den has to be placed in such a way that an ogre is certain to spawn within 4 tiles of the entrance used by the enemy.
It has not been exhaustively tested, but the nature of the AI targeting in Dungeon Overlord suggests that a creature that acquires a target will not switch targets until his current target dies. Therefore, a tank is useless if the enemies have already targeted your other creatures, and should always be first in a raid list followed by a delay of several rounds. Even then, slower creatures that serve a tank role can easily fall behind your other creatures during a raid and end up being too far behind to influence targeting, so they should not be viewed as a way to nullify all enemy damage.