Q: What exactly does 'Protection' enchantment do? Does it protect from falling? Fire? Explosions? Suffocation? How much damage does it absorb per enchantment level?
The Wiki says enough about other enchantments but for the 'Protection' enchantment it is somewhat vague:

Converts environmental damage to armor damage

And what happens if multiple armor pieces have this enchantment?

A: Protection will stack on different armor pieces, and provides comprehensive protection against all forms of damage except The Void and server kill commands.  It is true that falling, fire, lava, poison, and even mob damage will be reduced, but false that the armor durability will always reduce more than normal.  When falling off a building with Protection IV boots, the boots will not suffer any damage, producing the same results as feather falling III boots. The Armor will take more damage from Mobs and Players however.
It is possible to get feather falling IV AND protection IV boots, reducing fall damage immensely.  All other pieces of armor with protection will add additional damage reduction in the same way the boots would.  Protection appears to be one step down from specific defenses (ex: Protection III = Feather Falling II when taking fall damage, or Protection III = Fire Protection II when in lava).
TL;DR- Protection enchantments are slightly less effective than specific ones, but protect against everything at once.  The best possible armor would be:
Helmet: Respiration III, Aqua Affinity I, Protection IV, Thorns III
Chest and legs: Protection IV, Thorns III
Boots: Protection IV, Feather Falling IV, Thorns III
Note that Only highest Level of Thorns take effect if present on multiple peices of armor. Source

A: What they mean by "Converts environmental damage to armor damage" is that it takes some of the damage dealt to your health (falling, damage from mobs, fire, TNT, etc.) and puts it into your armor durability instead, giving your armor more work and your health less damage. 
NOTE: There are also Protection enchantments that are focused on one specific way of gaining damage.
Source: http://www.minecraftwiki.net/wiki/Enchanting

A: Protection will mitigate damage from mob melee attacks and falling damage. It should also help with suffocation and drowning.
Feather fall will increase the distance you can fall without suffering damage, but it doesn't actually reduce the damage you suffer (if you fall far enough).
In practice, this means it protects you from zombies, spiders (maybe poison also?), zombie pigmen, silverfish, endermen, and wolves. 
Skeletons deal projectile damage.

A: Protection also protects you from the damage dealt from throwing enderpearls.

A: What protection does is provides a small amount of protection from everything and regular attacks. But, the other enchantments provide a large amount of protection from one specific thing

A: Protection,is a very good enchant if you go adventuring,as it protects you from all kinds of damage,but it is not as effective as the real enchants,like in fire protection,it does protect you from lava,but not as good.

A: Protection protects you from everything other than poison, fall damage, and drowning. You can fall in a lava pit and sit there until your armor breaks! (That's why I don't think fire protection is good.)
