X is the storyof two groups of people involved in a battle to decide humanity's fate. On the one hand, there are the Chi No Ryuu (Dragons of Earth), the Seven Angels, whose purpose is to bring about the end of the world. Your token "bad guys", if you will (although I hope to convince you, by the time you're done with this site, that things aren't really as simple as good or bad). To make a /very/ long story story short (well, shorter, at any rate), the way they will accomplish this is by destroying the kekkai scattered around the city of Tokyo. Kekkai are a sort of protective energy fields, and Tokyo is built upon several of them. If Tokyo's kekkai are destroyed, then that will be end of civilization and humanity, at least as we know it.
So it stands to reason that the other group of people are the ones trying to prevent armageddon, right? Right. The others are the Ten No Ryuu (Dragons of Heaven), also known as the Seven Seals (the Seven Seals??!! As in, the Apocalypse?? Well, in a way, maybe). So while the Angels go around causing earthquakes and chaos and mayhem while trying to destroy the kekkai, the Seals go around trying to stop them. They can do this because each of them has the ability to create their own kekkai as well, which serve the purpose of protecting the city from the battles with the Angels.
Of course there's more to it. This is CLAMP, after all.
The Seals and Angels are each lead by a kamui. "Kamui" means either "one who represents the majesty of God" or "one who hunts the majesty of God". There is one original Kamui (whose name actually is Kamui), and he must choose whether he will lead the Angels or the Seals. Although he doesn't know it, he has an opposite star (a metaphysical double, if you will), and once he chooses a side, this person will become the other kamui and take his place among the other side.
I swear I'm trying to keep this simple. =^.^=