The responses to this thread are great demonstrations of ignorance, highlighting the need for the OP to post this thread.
What is annoying is to see stupid people arguing against what is plainly true.
There's no point in trying to argue about what you think a "tank" should mean when Riot has already defined what a "tank" means to them for the purposes of their classification.
From the wiki, which does a good job outlining Riot's definition of the various classes:
Tanks are tough melee champions who sacrifice damage in exchange for powerful crowd control. While able to engage enemies in combat, a tank's purpose isn't usually to kill opponents; rather,** tanks excel at disrupting enemies and diverting focus to themselves, allowing them to lock down specific targets (or several targets at once), as well as remove (or peel) threats from their allies.**
In addition to strong base defenses, tanks generally have a means of amplifying their tankiness even further with their abilities, and tend to fully invest in defensive items to maximize their resilience. However,** tanks lack the tools to truly succeed in single combat, and their influence is limited by their low overall mobility,** preventing them from constantly staying on top of their targets.
As tanks can handle burst damage very well, they tend to succeed against assassins and most mages, but their vulnerability to continuous damage puts them at a disadvantage against fighters and marksmen. Subcategories of Tanks are vanguards, and wardens.
Vanguards are offensive tanks. Vanguards lead the charge for their team and are specialize at getting action started. Their explosive team fight initiation seeks to catch enemies out of position while allowing allies to follow-up to devastating effect.
Wardens are defensive tanks. Wardens stand steadfast, seeking to hold the line by** persistently locking down any on-comers **who try to pass them. Wardens keep their allies out of harm’s way and allow them to safely deal with enemies caught in the fray.
Fighters (also known as Bruisers) are a diverse group of short-ranged combatants **who excel at both dealing and surviving damage. **With easy access to heavy, continuous damage (or DPS) and a host of innate defenses, fighters thrive in extended fights as they seek out enemies to take down, but their limited range puts them at constant risk of being kept at bay (or kited) by their opponents via crowd control, range and mobility.
Fighters tend to have an advantage against assassins, whose burst tends to fall short of killing them when unaided, as well as tanks, whose inferior damage allows fighters to eventually defeat them in duels, but often struggle against mages and marksmen, whose superior reach allows them to kite approaching fighters. Sub-classes of the Fighters are juggernauts, and divers.
Juggernauts are melee titans** who relentlessly march down the opposition **and devastate those foolish enough to get within their grasp. They are the only subclass who excel at both dealing and taking significant amounts of damage, but in turn they have a tough time closing in on targets due to their **low range and extremely limited mobility. **
Divers are the more mobile portion of the Fighter class. Divers excel at singling out high-priority targets to blitz toward, immediately forcing those targets (and their teammates) to deal with the diver’s presence. Divers are not as durable as the tanks or juggernauts of the world, but Divers can take their fair share of punishment while bringing enough damage to be a real kill threat if left unchecked.
Read and understand these definitions, and then you will understand why Shen, Rammus, and Malphite are Tanks - but Mundo, Darius, and Nasus are Juggernauts. And why Vi, Hecarim, and Warwick don't even qualify as Juggernauts but as Divers.
You'd have to be pretty ignorant to confuse a diver with a tank. But it's understandable why some might confuse a juggernaut with a tank.
The only thing a juggernaut and a tank have in common is they both take a lot of damage before dying.
The differences between a Juggernaut and a Tank are very significant:
-Juggernauts have more damage but lack mobility and CC so they are kited the easiest.
-Juggernauts have various forms of sustain but rarely have abilities that amplify their raw defensive stats the way tanks do, so they generally aren't able to absorb as much damage as a tank.
-Juggernauts generally don't have the tools for initiating a teamfight, setting up picks, or enabling their teammates to follow up. They lack both mobility and CC which are key components of doing that effectively. And Juggernauts definitely are not good at peeling for their teammates. But they can kill faster in a teamfight and do better at split pushing or solo fight scenarios.
When you say you need a tank for your team, what you are really saying is you need someone who can not just be a meatshield but also can initiate for the team and protect the weaker members from getting dived on.
A juggernaut can't do that. So don't pick them if your team needs a genuine tank. There is more to being a genuine tank than just being able to absorb more damage than the average champion.
are tanks.
Red 
are NOT tanks. These are champions who are taken for their ability to do damage by having one or two damage items in their build, while getting tanky throughout the game through defensive purchases and/or built in sustain. These champions are meant to be tankier while still doing damage because they are meant to be threats to priority targets. Most of them give up mobility and heavy CC in order to have that damage. I repeat, these ARE NOT TANKS.
