The custom stats are as follow:

Restoration / RES: Restoration increases the amount of healing given when using health and mana potions. In addittion, having higher amounts of restoration causes you to automatically refil up to 6 of each potion upon visiting the nexus.
Protection / PRT: Protection is a Shield which will protect you from all damaging projectiles, It can only soak as much as what it says. So if you have 131 it will protect you from 131 damage. This shield is not affected by defense.
Might / MGT: Might is critical strike damage. You gain 0.1 bonus critical strike damage per 7 points.
Luck / LUC: Luck is your critical hit chance. You gain 1% critical chance per 10 luck, capping at 100% critical chance at 1000 luck
Surge / SG: Surge is gained by killing enemies. You gain 2 surge per enemy that is killed near you. If you reach 100 surge, your protection bar is refilled. Depending on equipped items, surge can also have additional effects. Surge will naturally decay when enemies have not recently been killed.
Fortune: (only given by items): This will increase your drop chance by %1 per stat point