The Tree of Life is a symbol from the Kabbalah (Qabala), continuing ten spheres or energy centres, called Sephiroth (numbers):

1. Kether - the eternal, the spiritual perfection
2. Chokmah - Power of creation, energy, wisdom
3. Binah - Understanding, realization
4. Chesed - Condensation, the power of growth and stability
5. Geburah - Movement and changing, the power of destruction
6. Tiphareth - Consciousness and harmony, the balance of the middle, beauty
7. Netzach - Creativity and anarchy, sensitivity
8. Hod - intellect and logic, structure (Mercur)
9. Yesod - Reflection and imagination (moon and water)
10. Malkuth - the root, the origin, the subconscious

(for a little more just click on the single spheres in the Tree's image map aside)

Further, you might want a look on the Structure of the Tree of Life

One might compare the Sephiroth with the Chakras in Yoga. The mystic way of self-realization requires the rediscovery of all levels of one's own being, from Malkuth - the physical reality - up to Kether as the spiritual perfection. For the Tarot, the Tree of Life is much more, the universal basic on which the whole system resides. The 22 trump cards of the Major Arcana represent the 22 paths that connect the ten spheres of the Tree, the ten spheres themselfs are the basic for the number cards as they count (Kether for the Aces, Chokmah for the 4 Twos, Binah for the 4 Threes etc.). You can read a little more about this on the page 'Number cards'.

Also, the ten spheres correspond to the four elements as well as to the various levels of the soul and thus to the four worlds of the Kabbalah (see 'Basics').

For the elements, you see it like this:
Fire (world of Atziluth) - Kether
Water (world of Briah) - Chokmah and Binah
Air (world of Yetzirah) - Chesed, Geburah, Tiphareth, Netzach, Hod and Yesod
Earth (world of Assiah) - Malkuth

As for the levels of soul, the corresponding looks like this:

Kether to Yechidah (the universe which is above or better: around all four worlds), but already reaching over to Chiah
Chokmah to Chiah (which corresponds to Atziluth/Fire)
Binah to Neshamah (which corresponds to Briah/Water)
Sephirot 4 - 9 (Chesed, Geburah, Tiphareth, Netzach, Hod and Yesod) correspond to Ruach (Yetzirah/Air)
Malkuth to Nefesh (Assiah/Earth).

This might all sound a bit confusing at first, but in the end it is as logical as beautiful its structure and conclusion. To put it short, the way down the Tree from Kether to Malkuth is the development of the pure spirit to conscious physical existance - the way from Malkuth to Kether is the development from physical existance to spiritual perfection.

Both ways are good and necessary to realize and understand the own inner self. Some people spend the rest of their lifes climbing up and down the Tree, detecting amazing and unexpected views and thoughts every time.