Protection is Global Agenda's equivalent of Armor. The more Protection a player or pet has, the less damage it takes from attacks. While there are many sources of Protection, they are divided into a small number of categories.
In general, 1 Protection is roughly equal to a 1% reduction in damage from sources it applies to. The relationship between Protection and Health should be noted - the more Health a player has, the more valuable Protection becomes; The more Protection a player has, the more valuable Health becomes. This relationship also creates the synergy between Protection and Healing - Protection makes each point of Healing more valuable.
Physical Protection helps reduce damage from all sources. Originally, Global Agenda was to have multiple damage types, some of which were non-Physical, however the idea was dropped. As such, when something grants a player a boost to Physical Protection, it will help reduces any damage taken.
Ranged Protection reduces damage from devices that deal Ranged damage. Ranged damage is one of the most common damage types, devices that target a single player are typically classed as Ranged.
Melee Protection reduces damage from melee weapons, such as the Recon's Ghost Sword, as well as the Elite Assassin's sword. Melee damage is fairly uncommon in PvE, though Recons that specialize into melee attacks (using Skill Points) can be very deadly in PvP.
Area of Effect Protection reduces damage from explosive devices. This type of damage is ususally more bursty than Ranged or Melee damage, but is less common than Ranged damage. AoE damage is especially deadly to the less protected targets, as it's easy to be hit by an explosive that was aimed at the person next to you. In PvP, Assaults that have specialized into the Explosives Skill tree are capable of large amount of AoE damage, while Minion Sentinels and Minion Ballistas are the most common sources of AoE damage in PvE.