UPDATE: I have added information on paladin tanking. Added clarification and fixed some errors.
We have several level 70 paladins and many others have or plan to have paladin alts, so I figured it would be helpful for me to give an outline of this class. I would also like to note that I am not trying to tell anyone how they should play a paladin, I am just outlining what can be the more confusing aspects of this class.
The paladin, according to Blizzard, is a defensive hybrid. To me this means that the Paladin in the group should be capable of filling in as offtank/offhealer when needed.
In order to fill those roles though, its important to know what main talents in each of the trees are.
Holy: This is pretty much self explanatory. The talents in the Holy tree enable paladins to be very capable healers. (And if you have the gear, good damage also.) The talents any Holy paladin should have are: Divine Intellect, Spritual Focus, Healing Light, Illumination, and Divine Favor. The effectiveness of the other talents is dependant on whether you have spelldamage gear or not.
Protection: This is the paladin's tanking tree. Paladins generate the most threat of any class capable of tanking if you have Righteous Fury up and have the Improved Righteous Fury maxed. Other talents essential to a Prot paladin are: Redoubt, Blessing of Kings, Blessing of Sanctuary, Reckoning, and Holy Shield. Ardent Defender is very helpful if your the main tank. Avenger's Shield is pretty much just good for pulling.
Retribution: This is the paladin's leveling tree. Straight from the mouth of Blizzard. Ret paladins are most effective when geared like warriors. This means they have a hell of a time tanking and with a 5-6k mana pool are not good healers. Important Ret talents are: Benediction, Improved Judgment, Seal of Command, Conviction, Vengeance, Repentance, Sanctified Crusader, Fanaticism, and Crusader Strike.
The important thing to remember is spec the way you enjoy. Remember it is only a game. This is my spec if you want to know: http://armory.worldofwarcraft.com/character-talents.xml?r=Eldre'Thalas&n=Hadoredic
Seals: The paladin seal system is generally the most confusing aspect of the class, so I am going to go over each seal and its use in detail.
Seal of Righteousness: This seal generates the most threat and does the most damage if you have spelldamage gear. It always adds damage on each swing.
Seal of Command: This seal is best for Ret spec paladins. It is reliant upon a proc, and it procs more often on a slower weapon. This is why Seal of Command should ALWAYS BE USED WITH A 2 HANDER. Seal of Command works best with a weapon that has a speed of 3.0 or slower, and normally will not proc more than 8 times a minute. If you run Seal of Command with a 1 hander it rarely procs because of the faster speed of a 1 hander. If the seal does not proc it is not doing damage nor is it generating threat, which means you are doing little more than autoattacking.
Seal of Vengeance: This is the new "dps" seal. It is best used with a 1h in a long boss fight in which you need to conserve mana. It has a dot effect that stacks up to 5 times but it can expire quickly. It does not generate much threat so it should not be used for tanking.
Seal of the Crusader: This seal, when active, increase attack power and weapon speed. If judged, it increased holy damage taken by the target. This is a good seal to judge on bosses with high health if you have not judged Light or Wisdom already.
Seal of Justice: Limit's a player's ability to move when judged, also prevents mobs from fleeing.
Seal of Wisdom: When active, can proc and restore mana to the paladin. When judged on a mob, it can proc and restore mana to the entire group.
Seal of Light: Same as Wisdom but affects health.
Tanking: This is probably the second aspect of the class most are confused about. I am going to go over the basic method of how to tank with a paladin. Assuming that your are prot spec, this is how you should handle every pull. Activate Holy Shield, have Seal of Righteousness ready to be judged, and pull the target so the initial aggro is on you. Always keep Holy Shield up as much as possible since it generates alot of threat from being hit. Consecrate works great if the pull has multiple mobs. Paladins do have a taunt in the event they lose aggro but its slightly different from the warrior one. To taunt a mob off a player, target that player and hit Righteous Defense. This ability has a limited range and a 15 second cooldown, but it has the same effect as a Warrior's taunt.
I am going to skip over the Blessings and Auras as they are pretty self explanatory. Maybe this thread will make people less confused about the Paladin class.