In Christian lore, the Tree of Life is one of two trees standing at the center of the Garden of Eden (as described in the Old Testament of the Bible). It's fruit is believed to bestow eternal life upon those who eat it. Alongside it stands the Tree of Knowledge, which bestows knowledge and with it the knowledge of good and evil. The Tree of Life is often portrayed as an inverted tree with it's roots up in the heavens, with only it's branches reaching the Earth.
The Tree of Life is also referenced in Judaic mystical ideology in the form of the Kabbalah (a doctrine of esoteric knowledge concerning God). Here the tree is more well known as the Tree of Sephiroth, and is represented by ten Sephirah (the ten 'enumerations' or 'emanations' of God), each connected by a total of 22 paths.
The names of the Sephiroth are:
Keter (Crown), Chokhmah (Wisdom), Binah (Understanding), Chesed (Mercy), Gevurah (Severity/Strength), Tiphereth (Glory/Beauty), Netzach (Victory), Hod (Majesty), Yesod (Foundation), Malkuth (Kingdom).
The Tree of Sephiroth is organized into three distinct columns. The center column (the Pillar of Mildness) is associated with the air element and neutrality, balancing the other two columns. The right column (the Pillar of Mercy) is associated with the fire element and the male aspect. The left column (the Pillar of Severity) is associated with the water element and the female aspect (typically considered to be the more destructive aspect).
A particularly puzzling scene in The End of Evangelion shows an office with three towering windows and a large pendulum swinging back and forth. The symbolism in this scene is often attributed to these three pillars. Furthermore, this balancing act can also be symbolic of the three aspects of the human psyche as proposed by the Freud Psychology Theory, which in summary, states that the mind consists of there different forces, the Id, the Ego, and the Super-Ego. The Ego (one's sense of self) must balance the needs of the Id and the Super-Ego. This is a critical aspect of the Instrumentality process.
The Tree of Life has been interpreted in various ways, but is typically considered to be a sort of map or guide to meditation, to higher awareness, and/or to the history and future of mankind.
Neon Genesis Evangelion relies heavily on Kabbalah imagery. The Tree of Life can be seen in the opening sequence of the series, as well as on the ceiling of NERV Commander Gendo Ikari's office (his desk is located at the Keter Sephirah, which symbolizes the Crown). The tree can also be seen fully manifested during the Third Impact as depicted in The End of Evangelion. In the series, the Tree of Life is a critical part of the Human Instrumentality Project, and the means by which humanity is to be evolved.