A tank (or the "meat shield") is the type of champion that attempts to aggrovate or grab the attention of enemy champions in order to protect other champions on the allied team. Since this role often requires them to suffer large amounts of damage, they rely on large amounts armor, magic resist, and health points. This makes their item build very important, particularly building to counter heavy damage dealers on the enemy team. Most of the Tanks in League are are frequently very strong and slow/heavy.
The role of the tank is to get up in the face of the AD and AP Carries and force them to do damage to you. You want to do this only when your carries are in range to do damage to them. The key part of this statement is that your allied champions need to be in proximity for this to work. Simply going in and taking damage when your team cannot counter-damage simultaniously often results in a 4v5, and your team will likely lose the fight.
Another objective of the tank is to put the enemy spells on cooldown. If the enemy wastes a spell on you when you are initiating the fight; then that means it might be on cooldown and not be able to be used on other champions. This gives more time for your AD and AP Carries to dish out more damage to the enemy team.
Keys for Successful TankingInitiating Fights
Knowing when to initiate can make or break team fights for you. Generally speaking, it is very difficult to say when to initiate without knowing the team compositions on both the allied or enemy team. Rule 1:
Be sure that your allies are in close enough proximity to follow up on your initiate. Often times you will see tanks use flash and then another gap closing spell (like a Malphite ultimate or Amumu bandage toss); which then creates too far of a distance between your allied team and the enemies. The enemies can then just quickly focus you and kill you before your team arrives. This would be the wrong way to initiate. Rule 2.
Be sure that you pay close attention to the cooldown on spells for the enemy team (especially if they are spamming those spells in an attempt to 'poke' or harass you at a distance). Initiating quickly after an enemy champion tries to harass (i.e. Morgana uses Dark Binding (Q) and misses) can be very effective. You might want to initiate a milisecond after this, knowing that that spell is going to be on cooldown. Lead the Team
As a tank, you will be leading your team through the forest. You should always
be in front of your team. You will be about half a second ahead of your team while they follow closely behind. Sometimes I think of it as a team effort. Your team has to position themselves behind you so that it is worth while to for them to attack you while your team hits from behind instead of walking past you while you are attacking them. Not every tank is played the same way, but this strategy is generally the case. When to Initiate
It is impossible to say, in a general way how to initiate becuase every champion is different. Malphite's ultimate is obivous, Alistar would be Headbutt/Pulverize combo, Amumu would be a bandage toss to ultimate. Sometimes, instead of using your ultimate to initiate (this is more traditional in late game). It is better to just walk in and start taking damage and doing as much DPS as possible and then using your more traditional method of initiating later in the team fight (when team fights last much longer). This way, you're more likely have have already burned a few of the summoner spells and abilities of the carries. Again, this is highly game dependent.
Remember though, you don't always have to intiate becuase you are the tank. Sometimes there are other champions on your allied team that would be better off initiating, such as a Gangplank ultimate or an Ashe arrow. Furthermore, there are circumstances and map positioning where it might be better to wait for the enemy to initiate and counter their initiation tactics. Make Calls Early & Often
Think ahead, prior to a team fight breaking out. Map out the moves ahead of them happening, almost like a game of chess, and be a few steps ahead of the enemy team. If you make a call for a fight after something has already happened, you were too late. If you know where the enemy team is going to go, be ready for them at an objective. For example, when playing Soraka in a team fight, it might be temping to go ahead and silence the first enemy that comes into the fight. You must be patient and wait for that Kassadin to come in, then silence him before he's able to Rift Walk away. Common Tank Items
Counter Building to Teams
[Guide]How To Tank Guardian Angel
[Guide]How To Tank Warmog's Armor
[Guide]How To Tank Force of Nature
[Guide]How To Tank Randuin's Omen
[Guide]How To Tank Banshee's Veil
[Guide]How To Tank Thornmail
[Guide]How To Tank Aegis of the Legion
[Guide]How To Tank Atma's Impaler
Building the correct items is very important. You should never be building the same build on the tank every game. That's just mindless and not an intelligent way of playing the game. You want to ensure that you are countering the builds on the enemy team. If you see that the enemy AD Carry has 8 kills and their AP Carry has no kills, then you'll want to build armor, not magic resist. Think about how to maximize the disruption you cause the enemy team, and how to live as long as possible once you've started a fight.