Zammy and Sara ones have a 3-6% chance of lowered the damage dealt to you to 10% of the original damage if you're wearing 3-6 armour pieces (1% per piece if over 3 pieces, 2 = 0%).
Arma and Bandos work in the same way, but has a chance of lowering the cooldown of abilities with 30s or higher cooldowns by 15% I think.
The only difference between Zammy/sara is appearance.