Tanking is the act of sending one hero in to a base with the purpose and intent to destroy large numbers of units. This also involves the use of a Cloak Of Flames, as it's constant damage has an area of effect, which decimates all units below tier 3 and 4, resulting in instant, and large, cash bonuses should someone not have sent in their tier 0 footmen during the early game. Look for heroes such as the Blademaster, Halpmeh, Archmage and Bloodmage, as users of these heroes will spend more time with keeping thier heroes healed and working than sending in their units in the early game.
Such heroes are usually ones whose skills allow them to either absorb or return large amounts of damage, or allow for reincarnation. The Tauren Chieftain’s Reincarnation is the most obvious example of a reincarnation spell, but the Panda's Storm, Fire and Earth is a lesser version; by saving one of the three pandas which spawn from it, the panda is technically reborn from it, and usually out of danger.
The Metamorphosis skill, as used by Fox and the Demon Hunter is a very effective absorption skill, allowing the heroes to take higher amounts of damage, as well as allowing them to splash chaos damage to their enemies, resulting in faster kills. Coupled with a Necromancer's Unholy Frenzy or the Shaman's Bloodlust and lightning shield, along with a Mask Of Death, these heroes can remove almost any number of tier 0 to tier 2 units with little to no bother.
The Crypt Lord is also a very standalone tanker. While not having the blatant reincarnation of the Tauren Chieftain, or the visual demolition of the Demon Hunter, he is adept at survival in a melee combat surround situation. His Spiked Carapace is a local thorns aura and devotion aura, with its max level allowing him to remove the threat from almost any melee units when used in conjunction with his Locust Swarm.