Tarot cards are a medium of divination, which uses cards (similar to playing cards) to divine the past, present, future. They can also be used to read someone's personality, or to give them advice on what to do in life.
A deck of tarot cards contains the usual 52 cards (standard for a playing card deck) with an additional 26 cards, known as the 'major arcana.' The standard 52 cards are referred to as the 'minor arcana.' The major arcana include cards such as The Universe, The Lovers, The Tower, The Hanged Man, etc.
Essentially, depending on how you draw the cards and how they are placed in the 'spread' (layout of the cards), the cards will communicate different things, and thus give you a different reading.
In my personal opinion, Tarot cards are very reliable, as they offer a wide range of possibilities that can each uniquely fit the 'quarant.' (The person getting the reading)
If you'd like to learn more about giving tarot readings, the meaning of each card, and learn some tarot spreads to use, I highly suggest visiting this website. Simply click on 'lessons' in the sidebar and you'll be directed to an in depth course on the subject.