Magic: The Gathering is a card game with creatures. Both you and your opponent start out with 20 life and you try to reduce your opponent’s life to 0.
Card types include creatures, which you use to attack, and spells which have some effect on the game like you gaining 5 life or causing your opponent damage. Games include a lot of strategy which makes the games exciting, and helps make Magic infinitely enjoyable. Magic has been called “cardboard cool-aid” and “cardboard crack” because it is so addictive.
The official Magic website is here and an overview of Magic is here. A good demo for PCs can be downloaded here (85 megabytes).