Protection is an ability which essentially keeps permanents protected from whatever ability is stated, though the protection is not complete. An object is usually protected from a color, but objects can be protected from whatever the ability says it's protected from. Most instances of protection apply to creatures, with the exceptions of Eight-and-a-half-tails, Sygg, River Guide, Razor Barrier, and Seht's Tiger, so this article will only discuss creatures. The same things apply when other cards are given protection, but obviously not the effects that could only apply to creatures (such as equipment). Many believe that the creature Progenitus is the most powerful creature that exists, because it has protection from everything, making it almost unkillable, as well as having a massive power and toughness, (10/10) and will return to the deck when sent to the graveyard. However, casting it is nearly impossible because it needs 2 mana of each color ( ).
The abilities of protection are best summed up with the acronym D.E.B.T.
D For creatures with protection, all Damage that would be dealt to it from sources of that quality is prevented. For example, if a creature has protection from black, black creatures can't deal any damage to it, nor can any other black cards deal damage to it.
E The card cannot be Enchanted or Equipped by cards of that quality. Again, a creature protected from black cannot be enchanted by black enchantments or black equipment. If a creature gains protection from the quality after it has already been enchanted/equipped by that source, the attached cards will detach and go where the rules would normally force them to go in that circumstance. For this reason, white enchantments that grant protection from any color tend to state that their effects cannot remove themselves should you name white (or the enchantment's color is changed to whatever you named).
B Creatures of the stated quality cannot Block the protected creature. So, a creature with protection from artifacts could not be blocked by artifact creatures. This does not prevent the protected creature from blocking those creatures, but they cannot block it. For example, White Knight cannot be blocked by black creatures, but it's still free to block other black creatures when they attack you.
T The creature can't be Targeted by sources of the named quality. For example, Putrefy could not target something protected by black, green, instants, or whatever other qualities it happens to meet. This applies even if the creature didn't originally have protection, but gained it before the spell/effect resolved. If you tried to play Putrefy on an otherwise normal creature, and Bathe in Light was played in response to name green, the Putrefy would fizzle on resolution because its target is no longer legal. Keep in mind that this only applies for targeted spells/effects, which means it has to say "target" on it. If it doesn't target anything but just affects it, protection does nothing. Wrath of God is a common example of something that could get through "Protection from white", for example.
Color Pie PlacementEdit
Generally, white has the lion's share of protection. Sometimes the other colors will get protection from their rival colors, but green tends to opt more on its own "protection" mechanics; shroud and hexproof. Notably, blue also has a considerable amount of protection abilities, except it's mostly from things other than colors (instants, for example).
Comprehensive Rules Dealing with ProtectionEdit
702.15 - Protection
5702.15a - Protection is a static ability, written "Protection from [quality]." This quality is usually a color (as in "protection from black") but can be any characteristic value. If the quality happens to be a card name, it is treated as such only if the protection ability specifices the quality is a name. If the quality is a card type, subtype, or supertype, the protection applies to sources that are permanents with that card type, subtype, or supertype and to any sources not on the battlefield that are of that card type, subtype, or supertype. This is an exception to rule 109.2.
702.15b - A permanent or player with protection can't be targeted by spells with the stated quality and can't be targeted by abilities from a source with the stated quality.
702.15c - A permanent or player with protection can't be enchanted by Auras that have the stated quality. Such Auras attached to the permanent or player with protection will be put into their owners' graveyards as a state-based effect. (See rule 704, "State-Based Effects.")
702.15d - A permanent with protection can't be equipped by Equipment that have the stated quality or fortified by Fortifications that have the stated quality. Such Equipment or Fortifications become unattached from that permanent as a state-based action, but remain on the battlefield. (See rule 704, "State-Based Effects.")
702.15e - Any damage that would be dealt by sources that have the stated quality to a permanent or player that has protection is prevented.
702.15f - Attacking creatures with protection can't be blocked by creatures that have the stated quality.
702.15g - "Protection from [quality A] and from [quality B]" is shorthand for "protection from [quality A]" and "Protection from [quality B]"; it behaves as two separate protection abilities. If an effect causes an object with such an ability to lose protection from [quality A], for example, that object would still have protection from [quality B].
702.15h - "Protection from all [characteristic]" is shorthand for "protection from [quality A]," "protection from [quality B]," and so on for each possible quality the listed characteristic could have; it behaves as multiple separate protection abilities. If an effect causes an object with such an ability to lose protection from [quality A], for example, that object would still have protection from [quality B], [quality C], and so on.
702.15i - "Protection from everything" is a variant of the protection ability. A permanent with protection from everything has protection from each object regardless of that object's characteristic values. Such a permanent can't be targeted by spells or abilities, echanted by Auras, equipped by Equipment, fortified by Fortifications, or blocked by creatures, and all damage that would be dealt to it is prevented.
702.15j - Multiple instances of protection from the same quality on the same permanent or player are redundant.