What little I know.

The most important thing I know about Tarot cards is that you don't need to know about them in order to use them.
The tarot deck is like a language with 78 words. No two people agree exactly what each word means.
And the great thing is that they don't need to. The symbols on the cards speak directly to the parts of the brain that don't deal in words.
Everyone can therefore build their own relationship with the cards, without having to be "correct", so long as they are reasonably consistent.

Obviously for the sake of confidence and convenience it is useful to have a basic framework.
The deck is divided into 22 major cards and 56 minor ones.

The major cards are frequently considered to represent a cyclic progression of human development, for the individual, or the species, or both.
They can also be correlated with pathways on the quabbalistic "tree of life."
This in turn can be seen as a diagram of the human body and chakra system.
You can spend lifetimes working out exactly how you think it all fits together. People have.

The 56 minor cards are divided into four suits, Wands, Swords, Cups and Pentacles (or Discs) of fourteen cards each.
In each suit there are ten numbered cards and four court cards.
The four suits can be linked to the four elements: Fire, Air, Water and Earth.
Or to different aspects of human brain function.
Or, commonly, to intellect, emotions, sex and money, which may be the same thing.

So how does it work? How can a tarot reading tell people stuff? Partly because the weight of tradition and mystery encourages people to take it seriously and to concentrate. Partly because the vocabulary of the 78 cards allows nearly anything to be said that needs to be. And mainly for the same reason that allows divination of any kind to work: everything is connected. Everyone and everything pulses in and out of unconditioned emptiness trillions of times a second. There's no way we can keep up with this, nor would we necessarily wish to. But the moment when the human says stop and the cards stop being shuffled and are laid out, well, that moment is dripping with everything else that has just burst into creation once again. If a divination doesn't work it's simply that the attention of those involved could not be focused enough to keep out all the noise of billions of other beings and their concerns.