The history of the Tarot is unclear, there are various theories. Some people have tried to make a link back to ancient Egyptian magic, however the most widely held theory is that the Tarot originated in Europe, some time around the 14th or 15th century.
What is a Tarot Deck?
A Tarot deck consists of 78 cards divided into two sections: 22 Major Arcana and 56 Minor Arcana. The Major Arcana are the most important cards and are always illustrated, however the Minor Arcana are sometimes just shown as suit designs much like our modern playing cards. It's possible to buy so-called Tarot decks that consist of only the Major Arcana, however I wouldn't use these; if every card is special then no card is special.
There are a large number of different Tarot decks available to buy today, ranging from reproductions of ancient cards through to modern decks with very specific designs and symbolism.
How Does Tarot Work?
A Tarot reading usually consists of the reader laying out a spread of cards then interpreting them for the querent. Some people say the cards themselves have power, however this seems unlikely given their origin.
A more likely explanation seems to me that the cards and their symbols trigger something in the reader. This 'something' is as mundane as subconsciously picking up on clues from the querent or as deep as making a connection with the underlying reality of the universe.