Dark Lord of the Sith

I remember as a kid in my Star Wars storybooks, they referred to Darth Vader as a Dark Lord of the Sith; but they never explained what that meant!  It had such a cool air of mystery around it, who where these Sith? Were there more?  Did they all look like him?
Here is a recent commission I did of Mr. Darth "Annie" Vader.  Yes, I do commissions, my prices are $50 for a 9x12 Black and White and $100 for a 11x17 black and white.  Those prices are for one or two characters...more characters, the price is negotiable.