I play magic the gathering, and I want to know what a planeswalker is...?

Planeswalkers are the new "buzz" in MTG. I like to look at it as an extra place an opponent can place an unblocked attack and it absorps the damage instead of me. Planewalkers have only 1 number on the bottom of their card and it doesn't stand for attack or defense, it's a loyalty counter. Each ability can either add to the loyalty counters or take away from the loyalty counters, depending on what you want the planeswalker to do. Currently all planeswalkers follow the same pattern. The first ability is an ability that adds loyalty counters to your planeswalker. The second ability is a medium type ability that takes loyalty counters away from your planeswalker. The third ability usually has to be built up to and is known as the ultimate ability that takes a lot of loyalty counters off your planeswalker. You may only activate one ability of your planeswalker on your turn, as if playing a sorcery. The planeswalker is not a creature, therefore cannot block attacking creatures, nor can it attack. It is a permanent, an may be targeted by spells that target permanents. It does not go away if your opponent plays a Wrath effect. Opponents may remove loyalty counters in a few ways. They can attack with their creatures, but they have to announce whether they are attacking you or your planeswalker. Damage that goes to your planeswalker will remove one loyalty counter per point of damage. Damage that goes over the number of loyalty counters that the planeswalker has will destroy the planeswalker. Damage that goes over the amount of loyalty counters the planeswalker has does not trickle over to you, even if the creature has trample. So a planeswalker is also like having extra life if your opponent feels the planeswalker is a threat. Loyalty counters can also be removed by targeting direct damage to you, the opponent, and then redirecting the damage to your planeswalker. A shock, rift bolt or the like has to be able to hit you and then can be redirected to your planeswalker if your opponent so chooses. Like anything else, if the planeswalker takes the damage, you take nothing. If you have a spell or enchantment that gives you shroud, such as the creature True Believer or the enchantment Ivory Mask, then since the spells cannot target you, your planeswalker is safe as well. Like I mentioned previously, planeswalkers are targetable by Oblivion Ring, Root Grapple, or any other spell that targets permanents, so they can be removed from game or destroyed without being redirected through you.

I hope this helps and if you have questions, feel free to contact me.

I hope you enjoy playing MTG as much as I do.

From the MTG Wiki on planeswalkers:

212.9d Loyalty is a characteristic only planeswalkers have. The loyalty of a planeswalker not in play is equal to the number printed in its lower right corner. The loyalty of a planeswalker in play is equal to the number of loyalty counters on it. A planeswalker is treated as if its text box included, “This permanent comes into play with a number of loyalty counters on it equal to its printed loyalty number”; this ability creates a replacement effect (see rule 419.1). As a planeswalker gains or loses loyalty, loyalty counters are put on it or removed from it, respectively. Damage dealt to a planeswalker results in that many loyalty counters being removed from it. If a planeswalker’s loyalty is 0, it’s put into its owner’s graveyard as a state-based effect. See rule 420.5.