Tanks are durable, front-line champions that help lock down enemies and start fights. They're usually found leading the charge, choosing the right times and situations to initiate aggression. Many tanks can also protect their more fragile teammates by stunning or pushing around dangerous foes and limiting their damage potential.
These heavy-duty champions boast powerful defenses in addition to tremendous natural resilience (or tankiness), and while entering a fight is particularly difficult for a juggernaut, actual combat heavily favors them once they are in range. Juggernauts tend to favor heavily defensive builds, though what few offensive items they purchase significantly enhance their already potent innate damage.
The wiki is your friend, learn to use it when defining the purpose of champions and clarifying strengths and weaknesses of champions.
Tanks lead the fight, engaging and locking down enemies for the team.
Juggernauts follow up on the engage and attempt to kill enemies
Please refrain from assuming that because a champion is durable it means they are a tank, juggernauts are not tanks.