I noticed someone ask about Planeswalkers in the trading thread and I figured that it is best answered here in its own thread. Perhaps it should be stickied, but that's up to Malignant I guess.
What are Planeswakers?
Planeswalkers are a new card type (like Land, Creature, Artifact, etc.) that was hinted at in Future Sight (see Tarmogoyf) and unveiled in Lorwyn. They represent the ability to "phone a friend" when you are having a bit of trouble with your foe. An example of a planeswalker is shown below.
How do Planeswakers Work?
It is a permanent and can only be cast at sorcery speed.
It enters the battlefield with a number of loyalty counters equal to the number on the bottom right corner. For Jace, the Mind Sculptor, that is 3. If, at any time, a Planeswalker has 0 loyalty counters on it, it is put into the graveyard. (You've used up all your favours, as it were.)
It has a number of activated abilities that can only be used at sorcery speed and you can only use one of its abilities a turn. Hence, you can't use its first and second ability in the same turn. The numbers on the left hand side of the text box indicate how much loyalty counters to put onto the Planeswalker. For the above card, we see that, to activate its first ability, we put 2 counters onto Jace. To activate its second ability, we put 0 counters on Jace and so on. All costs must be able to be paid in order to use its ability. Thus, we can't use its final ability unless there are 12 or more loyalty counters on Jace.
Each planeswalker has a type. If there are two planeswalkers in play with the same type, both planeswalkers are immediately placed into the graveyard as a state-based action. This is very similar to the Legends rule. However, note that if there is a Jace, the Mind Sculptor in play and I successfully resolve Jace Beleren, both are put into the graveyard, whereas Akroma, Angel of Wrath and Akroma, Angel of Fury can both be on the battlefield at the same time.
Planeswalkers and Combat
If you control a planeswalker and I have a number of attackers, when I go to declare attackers, I can have the creature attack the planeswalker instead of you. You, then, choose who to block which creature normally. For each point of damage dealt to a Planeswalker, that Planeswalker loses a loyalty counter.
I think the best way to explain how this works is via an example.
Xavier is playing against Liann. Xavier controls a 2/2 Runeclaw Bear, a 3/3 Centaur Courser, a 1/1 Llanowar Elves and a 7/7 Enormous Baloth. Meanwhile, Liann controls a Liliana Vess with 6 loyalty counters on it and some number of creatures.
The following situations can occur.
1) Xavier attacks Liann with his Runeclaw Bear, Centaur Courser, Llanowar Elves and Enormous Baloth. It is like a normal combat situation without the Planeswalker in play. Liann can, then, choose which of these creatures to block.
2) Xavier attacks Liliana with all of his creatures. Only the Baloth gets blocked. Assuming no combat tricks, the Elf, Bear and Centaur deal a combined total of 6 damage to Liliana. This causes Liliana to lose 6 loyalty counters, putting her to 0 which, in turn, puts her into the graveyard.
3) Xavier attacks Liann with the Baloth and Elf and Liliana with the Bear and the Centaur. Liann decides to block the Baloth and the Bear. In this situation, the Elf will deal 1 damage to Liann and the Centaur will deal 3 damage to Liliana, putting her to 3 loyalty counters.
Planeswalkers and Spells
Another way of reducing the loyalty of a Planeswalker is through spells that deal damage to a player. If you control a Planeswalker, I can target you with such a spell and then redirect all
of the damage onto your Planeswalker instead. However, I cannot deal partial damage from the spell. Also, you cannot redirect damage from yourself to a Planeswalker. They are loyal, but they are not that loyal.
Time for an example.
You control Garruk Wildspeaker with 2 loyalty counters on it. During the end step, I decide to cast Lightning Bolt, targeting you. While the spell is resolving, I have to make the choice of either redirecting all 3 damage onto Garruk or leave it heading your way. I cannot have 2 redirected onto Garruk and 1 to you. It is an all or nothing affair.
To summarise, Planeswalkers are permanents that can use their special abilities during your turn at sorcery speed. To use their abilities, you have to pay the loyalty cost of the ability. They can be attacked as if they were a player and lose loyalty counters whenever they are dealt damage. In order to damage them with spell, you have to target the controller of the Planeswalker and redirect all of the damage onto the Planeswalker.
If you have any more questions, feel free to post in this thread. I hope this clears up a lot of problems and questions you may have with the new rules.