Planeswalkers have a "life" total which is instead called Loyalty.
They have abilities (which are played as sorceries and only one ability may be played per turn). Their abilities can either add or subtract from their Loyalty (the cost is listed before the ability (+1, -1, -10, -X, etc). If their loyalty goes below one they leave play. You may not spend more loyalty than they possess (so if a Planeswalker currently has 8 loyalty, you can't actiave an ability which costs 9 or more).
An opponent may attack a planeswalker instead of you, you may block the creature using your own creatures. Any damage dealt by creatures to it is done by removing loyalty.
If someone does damage to a player, they (the damager, not the one being damaged) redirect some of that damage to the planeswalker, removing loyalty from it.
Mythic rares:
WotC is trying to cut down on the size of expansions (that was their story anyways). They print sets in sheets of 110 cards (as I recall), Regular rares will show up 3 times on a sheet, while Mythic rares will only show up once. This cuts down on the appearance of these rares (with a purple rarity symbol). So, you'll be less likely to get one. The odd thing is, because of the way they print foils, you may be more likely to get a foil mythic than a mythic... There was much conjecture on how much these will price in the secondary market... I haven't looked at any of them to know an answer to that discussion.