One of the most important aspects of tanking is generating threat. This means you want the full attention of the big bad bosses so that they'll be hitting you instead of your friends. Some of the worst situations in a raid or dungeon come from the tank losing threat, so the boss or mob goes after someone else and causes a chain reaction of grisly deaths. You can call it the 'DPS pulling aggro', but the bottom line is that a tank's threat generation is what defines a group's strength -- the more threat you can generate, the more your DPS can go all out.
Threat and threat generation
What is threat? Most players should be familiar with this concept already. It's the measure of a mob's aggression, or aggro, towards a player. Every NPC has a threat list, and the person on top of that list gets beaten by the angry stick. That's about the basic thing. It's the tank's job for the most part to stay on top of that list. You can read more about threat and monster aggro in this Tank Talk article by Michael Gray.
Alright. So what tools does a Paladin tank have to generate aggro? Quite a few things, from the baseline Consecration, the various Seals (we'll get to those in a moment), Judgements, and introduced in Wrath of the Lich King, the Shield of Righteousness. Then there are Protection talents, the aptly named comic book-inspired Avenger's Shield, the 51-point Hammer of the Righteous, and the reactive Holy Shield.
Oh, and before anything else, you have to have Righteous Fury up. This buff increases the threat of your Holy spells -- which is pretty much everything you'll dish out -- by 90%. Many raids and parties have wiped because the tank has forgotten to throw up Righteous Fury. Don't be part of that errant crowd. On to the abilities.
Consecration is an AoE spell and one of the more familiar tools for Paladin tanks. It stays on the ground for 8 seconds, so dragging mobs over the Consecrated spot will cause the mobs to aggro you. The Wrath versions of the spell scale extremely well with both Spell Power and Attack Power, which means it generates respectable damage and consequently, respectable threat. The only drawbacks are that it breaks crowd control and it starts off pretty slowly.
Paladin tanks have a variety of seals to choose from, but Seal of Vengeance / Corruption is generally considered to be the best for tanking because it deals more damage (hence more threat) than Seal of Righteousness at full stacks. It also has a DoT component which lasts for 15 seconds and is refreshed with melee hits. DoTs stay on mobs, so they help keep you generating threat in cases where bosses wipe threat, and minimizes threat loss in case your attacks are avoided in some fashion.
On the other hand, the former tanking seal, Seal of Righteousness, frontloads more threat without requiring you to build up to a full stack. You can use this on trash with low life, or start off a pull to generate as much threat as possible then quickly switch to Seal of Vengeance or Corruption. Other seals simply aren't ideal for tanking, although Seal of Blood / of the Martyr generates recoil damage which can situationally help generate mana through Spiritual Attunement, soon to become a Protection talent. Heavy emphasis on the situationally.
You'll have three Judgements to choose from, but you're really only looking at Judgement of Wisdom and Light. If you have a Retribution Paladin in the raid, have her judge Light because it will scale better because they will generally have higher Spell Power + Attack Power. Wisdom, on the other hand, grants standard returns so this will be your primary Judgement. Some Paladin tanks won't give Judgements much importance because it doesn't deal much damage, but picking up one point in Improved Judgements will make it fit nicely into your rotation. It's a pretty good rotation, we'll get to that in a while.
Alright. Avenger's Shield is probably your primary pulling ability. It hits three mobs at the same time and dazes them as well. As you can probably tell, this isn't always a good thing. This makes it difficult to pull single mobs (although we now have Hand of Reckoning for this), and the slowing effect also makes it hard for us to start hitting the mobs right away to generate more threat. It's generally a good idea for ranged DPS to hold off on unloading nukes until mobs get within melee or Consecration range. Avenger's Shield is generally left out of a tank's normal rotation.
Shield of Righteousness is your biggest single target damage attack and generates the most burst threat of any spells in Protection's arsenal. This makes it part of your rotation and is a great way to even out threat on a particular mob you might not have built up enough threat on. This spell scales off block value and deal armor-bypassing Holy damage. It is the tank spell to use.
Hammer of the Righteous is your multi-target spell which works well in conjunction with your other spells. Even though it doesn't deal as much damage on a single target as Shield of Righteousness, it deals more damage and generates more threat overall when dealing with up to three mobs. The great thing about this spell is that it works off a weapon's DPS, so topend damage or speed doesn't really matter. Only marginally depending on your seal, but the idea is to get the highest DPS weapon you can find and whack with it. Hammer of the Righteous' secondary bounces do not break crowd control.
Finally, there's Holy Shield, which isn't something you use to hit things, but it's solidly part of your rotation. With the introduction of new abilities in Wrath, Paladin tanking is no longer as reactive as it used to be, so Holy Shield isn't quite as important. You will still need to have it up constantly as it provides solid mitigation. These are your basic tools for tanking. Next we'll take a look at the fun part -- your rotation.