Don't confused Juggernauts for Tanks
Juggernauts are tanky. So people often confuse them for tanks. But watching a lot of LCS and reading patch notes. It should become clear what the differences are between a Juggernaut and tank, as defined by Riot and high level players.
Juggernauts: They have a tanking steroid, they deal consistent single target or AoE damage, have mild CC, and remain a consistent raid boss style threat through an entire team fight or other engagement.
Tank: They also have a tanking steroid, however they typically deal much less damage(unless overtuned), have hard initiations tools, crowd control for lockdown and peel, and once they initiate they leave it up to their carries to do the damage.
Tanks can deal some damage, but not as much as a well built juggernaut. They also tend to lack the moments of burst that people like Trundle, Darius, Garen, Nasus have.
Juggernauts are tanky, but usually not as tanking.
Role wise the biggest separation is what they do BECAUSE of the differences in their abilities. If you take all the primarily listed Tanks in the game, you will notice almost every single one of them has one ability that lets them dive into the enemy team, and a strong crowd control or three. Meaning their job is to dive into a fight, crowd control as many enemies as possible, and peel damage dealers off of your carries.
Whereas most Juggernauts, have a slow and AS reduction, or a slow and pull, or a slow and pillar. And that is it. They also tend to lack tools that let them hard dive initiate. But they have great sustain, are also tanky, and have good damage usually on shorter cooldowns than a tank does. This lets the Juggernaut do his job as an off-tank damage dealer. He acts as a front line threat who who gets in there and deals enough damage to make themselves a relevant target, while being tanky so they don't immediately die. Their job is to lower HP bars and take damage.
The reason this distinction is important, is because when somebody posts that "Tanks do too much damage with 1 damage item." Then their examples are Garen, Darius or Shyvana. They are saying blaming tanks for a Juggernaut problem. If they say "Tanks die too fast despite building pure tank." Then name Darius, Garen, Shyvana or Nasus, they are saying tanks need a buff because of a Juggernaut problem.
It is also important because if your team has no CC and says "We need a tank." And you pick Garen, it's nice that you picked somebody tanky, but you still don't have proper CC. Some Juggernauts are almost tanks, like Trundle, Mundo or Volibear, and in some comps can fill the role of the tank if the team has enough initiation and CC already. But they are NOT tanks. And some tanks are almost Juggernauts, like J4 or Singed because they deal a lot of damage, but they are still NOT Juggernauts. They have different tools, and different jobs.
Before posting about Tanks or Juggernauts. Please make sure you are addressing the right one.
Sincerely, Bronzelord who plays Shyvana, Leona, Sejuani, Braum and Garen.
Sources: Riot, LCS, the Wiki, Champion GG, in game class tags, logic, friends who are much better than me.
Example of a pure juggernaut: 


?!?!?!?
(soon)
(sad but true)


is an Enchanter/Warden hybrid
, who are mostly ad items.
its tank assassin :3
perma Wither not a problem