The most important part is the Shield's skill cloud. It's abilities are as follows:
Name - Description - Stats Requirement
Block - For the next round, your shield takes 50% of the damage directed towards you. - No requirements
Intercession - Move between a nearby ally and enemy at melee range of each other, knocking both away from each other by 2 meters. - +3 spd, +2 str
Clipping - Swing the shield out in front of you for a chance to knock back an enemy 2 meters. - +4 str
Shield Smash - An overhead attack with the shield that hits for double the critical damage on a critical hit. +1 spd, +2 str
Burrow - Push your shield under an enemies guard to open them up for attack, enemy gains -20% armor for 2 rounds. - Body size must be at least one level lower than enemy's, +6 str
Long Yard - +33% intercession range for 1 round. - +4 spd
Double Body Blow - Use the edge of your shield to punch the enemy's body twice in quick succession, deals 2x normal attack damage. - +7 spd
Dual Shields - Allows you to wield two shields at the same time. - Medium/Large body size
Dual Block - Requires Dual Shields. Block with both shields at the same time for 2 rounds, each taking 50% of the damage intended for you.
Dangerous Impediment - Swing your shield(s) around in front of you to ward off enemies, lasts 2 rounds, knockback anyone within range 1 meter and makes 1 meter of ground in front of you impassable. - Medium/Large body size, +3 str, +2 spd
Crashing Symbols - Requires Dual Shields. Attack with both shields at once swinging them from opposite sides for +100 damage and a 36% chance to stun for 1 round. - Medium/Large body size, +5 str
Wall of Flesh - Open yourself up entirely to physically occupy as wide a space as possible, all attacks taken automatically ignore 50% armor. - Large body size
Crushing Blow - An overhand attack with both hands for 250 damage. - Large body size, +6 str.
Interceding Step - So long as two allies within intercession range are being attacked at melee range, intercede both enemies in the same round. - Large body size, +6 spd
The bull's head is also an excellent choice for a tank as it adds the hard head skill.
Hard Head - Head butt an opponent knocking them back 3 meters with a 20% chance to stun for 2 rounds.
There are two main styles of intercession tank, the knock back tank and the zone of control tank. Kb tanks focus heavily on strength, the idea being to intercede an enemy then physically remove them from the fight long enough for your back rows teammates to move out of the way or change the flow of battle. ZoC tanks on the other hand should be constantly moving. Obviously though you'll need to prioritize correctly so that you break enemy damage dealers off your squishier members, while not interfering in your damage dealers taking out their squishier members. Both are highly tactical positions, but the kb tank is more likely to make a single weighty decision every few rounds, while the ZoC tank is going to be constantly attempting to adjust the situation.
ZoC tanks are also very good for protecting sniper nests since the highly specialized units need to take almost no damage in order to survive. Kb tanks on the other hand are really at their best against other tanks, they can temporarily shut down a ZoC tank, or run merry havoc against cannoneers by preventing them from firing a loaded charge.
Both tanks benefit from size increases, though the strength based kb tank can also benefit from being smaller than an opponent due to burrow.
In terms of combo rotations, you'll really want to set up a first track of five very quick abilities starting with intercede. This way you can run a cycle against a single opponent then go back to the start to intercede another player. Kb tanks will want their second track to focus on knock back abilities, and third track to focus on utility, and maybe an extra intercede just in case. For the ZoC tank the second track should be all intercedes (you can change the first ability on the first track), and the third track should be utility and especially Long Yard.
Things to Keep in Mind
Make judicious use of Block. While there may not be any healers, there are channelers who can repair your shields in the field. Any damage directed instead to your shields could be a major advantage in a fight.
Bring spare shields. You never know when a huge attack is going to whallop your shields into submission. If you can pull out a new one and get right back in the fight, all the better.
Don't suicide lightly. Wall of Flesh can prevent a whole lot of damage from reaching your team, but it's not wise to throw yourself under the bus if you're the team's only or main tank. You'll have to weigh the effects of this particular barrage of damage, against how much you could save your team from by not taking it.