Lilith is the source of life for all creatures, including both Humans and the Angels. For the duration of the Evangelion she is kept in Terminal Dogma, where she is nailed to large red cross. The room where she is kept is marked as NERV's Main LCL (Link Connection Liquid) Plant--which is, in a way, a true label. Lilith is where NERV obtains its LCL, because LCL is actually Lilith's blood. The entry plug is filled with the oxygen-rich LCL, which takes the place of air in the pilot's breathing. Shinji notices that the LCL smells like blood, but he doesn't know why.
At the end of episode #16 Kaji and Misato mistake Lilith for Adam, the first angel. In episode #24 Kaworu also believes that Lilith is Adam, but upon closer examination realizes her true identity.
In #14 Rei is sent to stab Lilith with the Lance of Longinus, to prevent any further growth. But when the fifteenth angel, Arael, attacks Asuka, Gendo instructs Rei to retrieve the Lance to use against the angel. In the original television series the

removal of the lance causes no change in Lilith's condition, but in Evangelion: Death & Rebirth, the lower half of her body regenerates.

In the End of Evangelion, Rei joins with Lilith (after receiving the embryotic form of Adam from Gendo), and carries out the Third Impact. All human souls return to Lilith's Egg, but are soon returned to Earth due to Shinji's unwillingness to die.