The first series of seven comprises the Magician, the High Priestess, the Empress, the Emperor, the Pope, the Lover, and the Chariot. They can be seen as cards of being. We can become any one of those characters and take on their roles. They have the simplicity of intention and a personal power that equates with basic human drives.
The second series contains Justice, the Hermit, the Wheel of Fortune, Strength, the Hanged Man, Death, and Temperance. These can be interpreted as cards of interaction; the emphasis is on how we respond to the world and external forces, how we balance their energies and adjust to their demands. All these cards represent a choice, a way in which we can act in handling situations.
The third series consists of the Devil, the Tower, the Star, the Moon, the Sun, Judgment, and the World. Here, the emphasis seems to shift to external forces that affect us. These may be cards of higher energy that come from a different realm; note that the sky is a prominent feature in these cards, with the exception of the Devil, whose world anyway has no sky as we know it.
In this interpretation, then, the first series is the most personal, the second encompasses our dealings with the world and our fellow human beings, and the third denotes events and forces emanating from another level. This analysis is not the sole possible interpretation, but it may serve as a useful guide to seeing some kind of progression through the pack, with three marked stages and a consistency of imagery within each set of seven.