Well, it's your story, so you can take creative liberties and give them some variations and stuff, never limit yourself to just the facts. But to give a brief rundown, they kept track of the date in older times, were responsible for handling spiritual issues, and were famous for their shikigami techniques (think magic familiar, but made of paper or wood).
The discipline itself is called onmyōdō, I think. Something famous they used was (like we see often in bleach) a five pointed star. They also used hand symbols called Kuji-in, and were known for using divination and astrology. The five elements they used were, I believe, wood, water, fire, earth, and metal. Those elements are each represented as one point of the star they use. Most of their abilities come from rituals, and are mostly focused on appeasing, sealing, or warding off spirits. They also used talismans.
Of course, I'm not an expert, so all my information could be a bit off, but I think I got the basics here.