In a classic Tarot deck, there are 78 cards, divided into two main groups, the Major Arcana, and the Minor Arcana.
The Major Arcana is composed of 22 cards, numbered 0 to 21. These are the cards that show influences that the person asking the question (I call her the Seeker) has little or no control over. They represent power and mystery, and follow Jungian Archetypes. Sometimes they are seen as The Journey of the Fool, or the soul's evolution from ignorance into enlightenment. These are The Fool, The Magician, etc.
The Minor Arcana is divided into 4 suites, each composed of 4 court cards and 10 cards numbered one to ten. The court cards include a King, Queen, Knight and Page. The suites are Pentacles, Wands, Swords, and Cups.
Non-classic decks can vary widely from the classic ones, with differing numbers of cards, different suites, different court cards....you name it, it's been changed in some deck or another. In fact, some of these are so different from classic Tarot decks that there is spirited discussion about whether they are really Tarot at all!
No one knows for sure where the Tarot came from, although there are many theories. The first decks we still have seem to date from the later half of the fourteenth century. Anything else is just speculation. But there is plenty of speculation!
The Tarot is used by concentrating on a question, shuffling the cards, and then laying them out in a pattern, called a spread. There are hundreds of different spreads, and hundreds of different reading styles; slightly more than there are readers, in fact!
I think the Tarot is about more than predicting the future. I think it's also about finding out how you are doing now. If you let it, it will show you what sort of path you are on, and what shape that path is taking, as well as where it's leading you. That's part of the reason why so many of the cards in most spreads deal with the past and the present.
I think that what the Tarot does is shed a bit of light on the path, so the seeker can see where he's going, instead of wandering blind and simply hoping for the best.
If you want to know more, check out the individual topics to the left.