Readers describe the practice in two broadly different ways, and it is worth keeping them apart because they make different claims.
The divinatory account treats the cards as informative about circumstances the reader could not otherwise know, whether by chance guided from outside, by synchronicity, or without any offered mechanism. The reflective account, now probably the more common among professional readers, treats the deck as a structured set of archetypal images whose function is to prompt the querent to articulate what they already know, with the randomisation serving to break habitual framings rather than to transmit information. Neither account is the whole field, and many readers move between them.
The mechanics are simple. Cards are shuffled, cut and laid in a spread in which each position is assigned a meaning, past, present, likely outcome, what obstructs, what helps. The three-card line and the ten-card Celtic Cross are the most common; readers also design spreads for a particular question. Reversals, cards drawn upside down, are read by some as inverted or blocked meanings and ignored by others, and that disagreement is longstanding.
Where readers disagree most sharply is on prediction. A substantial part of the professional field now declines to make specific predictive claims at all, both on principle and because in several jurisdictions doing so for payment attracts legal restriction. Others read predictively and always have.
The one claim the historical record does support is that the tarot has become the most widely used symbolic vocabulary in Western esotericism, the common language into which astrology, Kabbalah, alchemy and elemental theory were all mapped in the nineteenth century, and the reason those systems are now so often encountered together.