"They" are basically zombies; reanimated corpses that were previously killed by "Them". "They" must bite someone to successfully turn the person into "Them". The main characters decide not to call them zombies because it is happening in real life, and not the movies, and has the added benefit of allowing them to disconnect themselves from being human more effectively. "They" cannot see and cannot feel anything. "They" follow any sounds they hear except for each other. "They" can also only be killed by having their heads severed or destroyed.
Nothing is known about how the infection came to be as of yet.