I'd just like to add a little to discofever's writeup.

While all the sephiroth are considered somewhat spiritual, the triangles mentioned by discofever are correlated to different "levels of consciousness," for lack of better terminology.
The supernal triangle can be seen as purely spiritual,
the ethical triangle represents the subconscious mind,
and the astral triangle may be seen as the conscious mind.

Sometimes Malkuth is considered part of the astral triangle, although it bears a stronger correlation with the body and physical surroundings. I've even seen a text saying to read a physics or chemistry textbook to gain a better understanding of Malkuth.

One correction I would like to make is that most of what I have read indicates Da'ath(knowledge) is not an eleventh sephira, but a "hole." According to those sources, the biblical fall of man resulted in a loss of direct knowledge of divinity (expulsion from Paradise), and the Kabballistic Tree of Life was restructured.