The Resistance is one of the two primary factions in the Future War; an heroic organization which fights against Skynet after Judgment Day and serves the main protagonists alongside John Connor, Kyle Reese and Sarah Connor. It is composed of an alliance of surviving professional military units and militias from all nations around the world. It's led by John Connor, who Skynet has been trying to kill from back in time to prevent the creation of the Resistance. Its main purpose was to fight the Skynet and its troops, and to preserve the human race from total annihilation. The Resistance carried out many combat missions attacking factories and breaking humans out ofSkynet Work Camps. To even the odds, they steal weapons andreprogram Terminators to help them. Resistance soldiers wear red bands on their arms to symbolize blood. The symbol of the Resistance is a Red Double Helix (the shape of DNA).
(copy and pasted straight from the terminator wiki.)