The Light is a force of the universe, operating in direct opposition to the Dark. The task of the Light is to keep the Dark from rising and creating a world in which joy, love and laughter can not exist. It is difficult to say, from the book, how much the Light is sentient and transcendent, able to think and act and desire for itself, and how much it is simply an idea or an influence that causes its allies to do what they think they should do.
In many ways they are like the Dark, being hard and cold, separate from the world of men and bound by the Laws which regulate the magics used. Both sides use agents of various kinds and can be ruthless in their goals. The Light will do what it has to to defeat the Dark—for instance, risking the lives of mortals—and some men and allies of the Dark point out that the Light is in some ways not much better than the Dark; for instance, John Rowland, after being told that the triumph of the Light is more important that one person's life, states that he'd rather save one person's life. Interestingly, this feeling plays an important role in the final conflict. However, unlike the Dark, the Light does not wish to destroy the world.
It is important to note that the Light and the Dark are not the only forces at work in the universe; there is also Wild Magic, the magic of the natural world, and High Magic, which governs the universe. All four forces are governed by rigid laws and exist seperately from each other, although certain of their laws and customs involve more than one kind (for instance, the challenge in the fifth book, in which the Dark and the Light both argue sides of a case; once a decision is made, it is ratified by the High Magic). There are also places, such as the Lost Land, in which neither Dark nor Light has power. There is a final element at work on Earth, and that is men. Men are inherently neither of the Light nor the Dark, although they can have good or evil in their hearts; the Dark and Light can use these inclinations to their advantage.
The Light sometimes manifests itself in joy, as well as in a certain chiming music that Will Stanton frequently hears but can never quite remember.
Many mortals and men Serve with the the Light despite Man's tendency to be a mix of Light and Dark. There wer the great craftsmen who created the things of Power, including Gwyddno Garanhir. There are the warriors, such as Arthur, and his son Bran. There are those like Hawkin, who are servants and friends. The Drew family were adventuresome children eager to work for the side of good. The Sleepers, probably knights of Arthur, are men who await the appointed time to awaken from their rest and fight as warriors again. Gwion works for the Light, but his loyalty is more with his king. Man's best friend, or dogs, seem to side with the Light, Rufus and Cafall being prime examples. Even the good, but neutral John Rowlands performs a service on the side of Light, by holding Bran's Sign.