Juggernauts deal a lot of damage, while having low mobility or at least enough to do their job but are susceptible to being kited, have little CC and are tanky. I think that is all the things that go into factoring a juggernaut.
Tanks don't deal a ton of damage, have a lot of cc, some of them can be highly mobile with moves that are also tied into CC moves, and focus on peeling and building as much stats to stay alive, no damage items unless you count Cinderhulk or Deadmans.
Though I do think juggernauts still need tweaks with either their items or ability to soak damage, while also dealing a massive amount. Things like Garen healing, damage reduction, Darius heal, and Skarner shielding gets really obnoxious when they are also destroying with how much damage they bring. Something will probably change or happen to that though. Tank items and tanks a whole have had nothing but nerfs this season though.. and that is an issue.