A Tank's job is to have the enemy team focus their spells and attacks on them so their teammates can do what they need to do without getting utterly owned.
A couple of Tank champions are:
DR Mundo
Rammus
Galio
Amumu
DR Mundo, The Madman Of Zaun. His abilities are: Infected Cleaver: Dr. Mundo hurls a cleaver in the target direction, dealing magic damage.
Burning Agony: Dr. Mundo hurls a cleaver, dealing magic damage to the first enemy it hits.
Masochism: For the next 5 seconds, Dr. Mundo's next basic attack deals 5% of his maximum health as damage.
Sadism: ( ult ) Dr. Mundo regenerates health every second for 12 seconds and gains bonus movement speed for the duration.
Rammus, The Armordillo. His abilities are: Powerball: Rammus tucks into a ball and begins to roll for up to 7 seconds, losing the ability to use basic attacks.
Defesive Ball Curl: Rammus drops into a defensive stance, gaining bonus armor and magic resistance.
Puncturing Taunt: Rammus Taunt icon taunts the target enemy and reduces their armor.
Tremors: ( ult ) Rammus creates an earthquake around himself, dealing damage to enemies each second for the duration.
Galio, The Sentinel's Sorrow. His abilities are: Resolute Smite: Galio fires a concussive blast at the target area, dealing magic damage to enemies.
Bulwark: Galio grants the target allied champion or himself bonus armor and magic resistance for 4 seconds.
Righteous Gust: Galio unleashes a gust of wind in the target direction, dealing magic damage to enemies.
Idol Of Durand: ( ult ) Galio turns into a statue and channels, halving incoming damage and taunting nearby enemies for the duration.
Amumu, The Sad Mummy. His abilities are: Bandage Toss: Amumu tosses a bandage in the target direction that stuns the first enemy it hits.
Despair: Amumu cries a continuous pool of tears, dealing magic damage every to nearby enemies.
Tantrum: Amumu releases his anger, dealing magic damage to nearby enemies.
Curse Of The Sad Mummy: ( ult ) Amumu deals magic damage to nearby enemies and Disarm iconRoot icon entangles them for 2 seconds.