Forgive any errors in this node - I've just began reading about the Kaballah, and it's a deep subject. You can't rely on Neon Genesis Evangelion for a good overview of this subject, kiddies.

Speaking of Evangelion, you can see a picture of the Sephiroth in the opening credits of said anime, and on the ceiling of Gendou Ikari's office. (It's entirely possible that the Kaballah and the Sephiroth have much more to do with the series; I've only viewed the first 10 episodes.)

Now, more substantial noise. The Sephiroth is laid out in a tree; hence, the easier English name, the Tree of Life. It describes the 'descent of divinity into the human realm'; sort of a trickle-down theory of divine grace. It starts at the head, the Kether (or crown) and reaches the Malkuth (the Earthly Kingdom). Here's the whole list of the sefirot (or sephira) :

These separate virtues aren't all really separate, supposedly; it's a package deal. Divinity generated from the ineffable will flow downward through the tree, energizing each sefirot like so many LEDs. Divine grace joins wisdom joins understanding joins mercy joins... And this analogy extends one step further; divinity flows down to us groundlings, not up from us. Those LEDs, they are diodes, and the Tree of Life is a one-way street. Trying to force power up the tree will lessen each sefirot. It won't care if you just became a Zen Master last night, your understanding won't translate to others until they are so blessed. It's not unlike enlightenment.

Each sefirot has two relationships. The first is a duality. Kether to Malkuth, Chokmah to Yesod, they are not opposites, but rather complementing halves. It's related to a male-female relationship. And then there's the standard religious trifecta. The number three pops up in many religions (the Holy Trinity, Shiva, Brahma and Vishnu, etc.) and the mystic Kaballah is no exception. Here, the Sephiroth is divided into three groups twice over...

The branches are mini-trees; as each sefirot has divinity flow through them from Kether to Malkuth, it also flows down directly from Binah to Geburah to Hod. And divinity flowing up from Hod will both negatively affect Netsach and Geburah. At this point, I'm not sure what the triangles are for.

Now this is all well and good, but what does it all mean? That is deliberately left as an exercise to the reader. In keeping with the mysticism of the Kaballah, they give you the tools, but don't say how to use them; supposedly, your intuition shall lead the way. Practically, this boils down to the following rule of thumb : if you think the Sephiroth may apply to a given situation, then it does.

There's one more sefirot, inside of the tree but not connected to any of the others. It's called Da'ath, or knowledge, and it's totally separate from the Sephiroth; at times, it's considered to be Binah and Chokmah blended together.