This division had always existed between the Others. For many millennia, the two sides fought a vicious battle. Both were willing to use any means to ensure victory. But eventually they realized that if they continued their battle, neither side would survive. So the leaders of both sides entered into a treaty - a set of laws to govern the way the Others used their powers. The Light Others created the Night Watch, to ensure that the Dark Others would not violate the Treaty. The Dark Others created the Day Watch, to ensure that the Light Others would not violate the Treaty. And the Inquisition, a group composed of both Dark and Light Others, was created to ensure that neither side became too powerful.
Since then, the two sides have kept an eye on each other and a fragile state of detente has developed. However, both sides are attempting to find a way to become powerful enough to discard the treaty and destroy the other.
Within the Gloom an Other becomes invisible to humans. Also, time flows at a much slower rate, giving them increased speed and a faster reaction time. While in the gloom, an Other's energy is gradually consumed. If they allow it to be exhausted, they are unable to leave and are sucked to the bottom levels, effectively dead.
It is said that all Others go to into the Gloom when they die, although meeting one of the dead ones there is very rare.
The Gloom has multiple "layers". Each step deeper, an Other moves further away from the conventional reality, as the environment around them changes from resembling the real world to something completely different. Also, the time dilation effect increases with each step and so do the strength requirements. As an Other's strength increases, their ability to reach lower levels does as well.
Vampires and werevolves can initiate both humans and Others by feeding on them. Here the choice is between the Dark side or death. Both vampire and werewolves are inherently Dark. They are usually regarded as lower Others, because anyone can become a vampire or werewolf by being bitten (one cannot, however, become one of the "regular" Others without being born as one).
Others can see a person's aura, which shows emotions and possible Light/Dark alignment. The aura can then be sent to other Others, and used to identify that person later.
Most Others have precognitive abilities, allowing them to see and somewhat alter the probabilities of events happening. This can be used, for example, to avoid traffic accidents when speeding. Grand Mages like Geser and Zavulon can predict some events day or even weeks before they happen, allowing them to forge complex plots that stretch far into the future.
Others can also drain Force directly from humans, Light Others by sapping the positive emotions and Dark Others by taking the negative ones. The difference is that the positive emotions disappear when taken, while the negative ones get worse. This usually makes this method unacceptable by Light Others.
They cannot lie outright, although they can conceal information and manipulate the truth.
Light Others also have a kind of "fail-safe" mechanism: if a Light Other cannot justify his actions to himself, he instantly dematerializes(?), banished forever in the Gloom.