For most people Fate/Stay Night is an anime, and indeed there is an anime adaptation of it out there. Yet before that there came something else. Fate/Stay Night started off as a visual novel, and is a product that has yet to see an official release outside of Japan. This, my friends, is a real shame, because after going through every route in this novel I have come to one conclusion - Fate/Stay Night is an amazing visual novel and a fantastic representation of what the genre can bring to the consumer.
Before going any further it should be noted that this genre is one that relies a lot on text displays to get the entertainment value across, and FSN has not been released outside of Japan. This means that unless you happen to be fluent in their language then you likely will miss out on a lot of the impact this title has. Thankfully there is a fan translation patch out there by the people at Mirror Moon that converts it all into English, and you'll need it.