Wikipedia defines a tank (gaming) as: "In gaming jargon, a tank is a player or unit designated to protect weaker players or units in combat by drawing enemy attacks and absorbing damage. Compared to other units, tanks have a high damage tolerance and specialize in melee combat. The term meat shield is roughly synonymous with tank, often carrying the additional connotation that the unit is relatively expendable or inconsequential and its death is preferred to the death of the units it protects."
Tanks are one of the most important players in a team, and without them the team will easily fall. A team needs atleast one tank. So let's analyse tanks to gain a better understanding.
A common misconception among DotA players is that the only tanks are the high-STR heroes with massive HP. They tend to overlook other key factors like armour, evasion and overall EHP (effective hitpoints, which factors in both armour and evasion)
They believe that tanks are solely for taking the damage, and are usually incapable of dishing out damage and carrying the team.
Sure, the Wikipedia definition is true... tanks must have high health, must be capable of taking in damage, and must die for the team. That's what we all want -- our team's tank taking all the punishment for us so that the lower-HP heroes can survive and attack the opposing heroes. And that's what we believe our tanks are doing... stocking on HP items before even considering what would be the best way to help the team.
This, is the ideal concept of a tank. But no one, and I mean NO ONE who's decent at DotA will target and focus fire on a tank UNLESS he's of some threat or hindrance to them.
I would like to partially rephrase the 'tank' term to suit the game of DotA - 'A tank is someone who can take the punishment for his team, acting as a meatshield and getting focus fired but at the same time can deal significant damage to the enemy.' A tank's presence must be felt. He must have the ability to last long in battle and contribute to the team as much as possible. Getting focus fired means that you're buying more time for your team mates to attack your enemies, and possibly saving your main carry.
This is why it is advisable to buy a Radiance for ALL TANKS. It's a key item for tanks and is what allows you to take the role of a tank. Immolation damage is invaluable to your team and I believe that there should be one on your team in almost every game. This is because with immolation, people will target you first (which is what you want) because it damages them over time. Coupled with your stuns/disables, you can really cause problems for them. Almost always, you should get it before a heart. Because if you get a Heart first then you would delay the Radiance taking it into late-game where it's not as effective as in the mid-game stages. Also, most tanks have some form of block damage either by skill or item, which helps them to play some mindgames coupled with their strong HP regen. Life-steal and the uncommon evasion on tanks (ex. Panda) also helps.
That said, a tank doesn't always have to be a STR hero. Even late-game agility carries like Troll Warlord, Faceless Void, Phantom Lancer, Mortred can become tanks in some cases because it's hard to bring them down with physical attacks (with the exception of magic/exact damage, which in fact tells us about the usefulness of INT heroes late game) because of their high armour and excellent evasion.
With this, it's easy to understand why heroes like Centaur, Axe, Bristleback*, Syllabear*, Davion, Leviathan, Sven, Panda, Balanar, Barathrum are commonly picked tanks. If you notice, ALL of them are also capable of carrying the team if the team requires one.
*Bristleback: Even after the nerfs (STR gain decrease), Bristleback remains one of the best tanks. This is because of his incredible ability to stay in battle because of his BB skill, and stouts/vanguard which you'll buy for him. He gets focus fired, but can still deal damage in an area with quillspray. Coupled with Radiance, you can dish out lots of damage in an area. And his STR gain nerf is nothing to worry about, because in the end, its just about a 10 STR less at level 25 (190 HP less, 10 damage less, and a small decrement in HP regen). He still has his awesome skills.
*Syllabear: After he gets True Form at level 16, he gets a MASSIVE 1k HP boost along with the skill "One" which enables him to share half the damage received with his bear. And you can keep it on endlessly. It's really hard to bring this guy down once he's fully farmed.
Oh, and tanks are fun. =)