Here are the rules that I got off the New York Times website.
A 'Set' is 3 cards in which each individual feature is either all the SAME on each card... OR all DIFFERENT on each card. That is to say, any feature in the 'Set' of three cards is either common to all three cards or is different on each card.
The Features are:
Symbols: Each card has ovals, squiggles, or diamonds on it; Color: The symbols are red, green, or purple; Number: Each card has one, two, or three symbols on it; Shading: The symbols are solid, striped or open.
I have found two different places on the internet to play, the official Set website and the New York Times. The great thing about the puzzle on the New York Times site is that you can play at two different levels, thus making it easier for beginners. The official Set website has only one level.