Bear in mind that here we want to talk about Paladin aggro mechanics and tanking at that moment. For most of the World of Warcraft Paladin players, they can not wait to take a quick look on it now, so let us get to the business. The details are as follows:
Aggro stands for mob aggressivity. In other words a player "has aggro" when a mob is focused on attacking the player.
The mobs never attack players randomly but base their target choice on a so called "aggro table". The aggro table is a ranked list based on the threat level every party player have on that specific mob. Mobs (with few exceptions) attack the player ranked 1st on the aggro table if they can. In other words they attack the player percieved as the biggest threat to their life. Normally threat vs mobs is increased dealing damage to them or healing a mate. However there are a lot of abilities in game that can raise or lower your threat greatly.
When you get too close to an hostile mob you get aggro. The distance when the mob is aware of you and attacks is called aggro range. The aggro range may vary depending on the difference of levels between mob and player. If the mob has an higher level than player the aggro range is wider, while if the player is higher level than the mob the aggro range is smaller. Now you know why you can't have your level 10 friend in your group when you enter stratholme instance for example. As soon as he enters a lot of mobs are aggroed from kilometers because vs a level 60 mob his aggro range is incredibly wide.
To large scale tanking is the art of keep mob attention. It's all about the threat the mob percieve. It requires you know all the tools your class have to raise your threat vs mobs. And also requires your fellows knows how aggro works and try to deal maximum damage/healing without attracting mob attention.
Paladins have seal of fury and judgement of fury. These 2 abilities work (like every other seal and judgement) on single target. They don't have any physical damage effect but simply influence aggro
If you used seal of fury with a fast one hand weapon, every time you hit you get 90 extra threat point in addition to your damage indipendently how much the damage was. Your threat points could easily double your real damage. But judgement was incredibly better. Every time you dealt holy damage you get 337 extra threat points indipendently how much the damage was.
If the mob is undead / demon type, exorcism is a great option for tanking. Its mana/damage ratio is far better than other abilities like hammer of wrath, holy shock and consecration.
For what regards multi target tanking you need absolutely Consecration (talent in holy tree). Also Holy Wrath could be used for multiple undead / demons tanking but its long cooldown (1 min) don't allow this tool to be reliable in keeping aggro.
Paladins gained the ability to tank multiple target but their effectiveness in tanking is lowered at minimum levels. Paladins can now play tank role in 5-10 man instances at max if no Warrior is avaiable but even a Druid in bear form can perform better than us. Why developers have done this is actually a mystery. I can understand the introduction of Righteous Fury as an easier to use aggro tool. A lot of Paladins never understood how Seal and Judgement of fury works and never use it, and complained on the forums how they couldn't keep aggro with their low damage but the threat we can generate now is pretty ridicolous, even with Improved righeous fury talent.