Evangelion(Or "Eva") are giant humanoid robots used to fight against the mysterious "Shito" (or "Angels").
An Eva can generate its own AT field, and carry weapons which can neutralize the AT field of Angels. However,
they are extremely costly to build and maintain. They need an external power unit in order to fight for an extended period of time.
Without an extrenal power source, it's maximum operating time without turning on the AT field is five minutes.
Eva units in service at NERV Headquarters are Eva-00, Eva-01 and Eva-02. Another Eva unit, Eva-03 is being tested in Japan,
but it had a major malfunction during testing in episode 18, and had to be destroyed by Eva-01.
EVA-00 was the first functional EVA unit. EVA-00 has one eye sensor in the front and a large one on the top of its head. EVA-00's internal structure is exactly the same as EVA-01 and EVA-02. EVA-00 was damaged when it went berserk during a synchronization test between it and its pilot, Rei Ayanami. The origianal unit 00 was painted light orange and pale yellow, however the current unit is painted blue and white.
EVA 01 is the most advanced unit. It has two main eyes. Its internal structure cockpit and weaponary are of human technology. Just like the Angels, it can modify those who were damaged. The cockpit is situated in its torso and is filled with a liquid that provides the pilot Shinji Ikari with oxygen and a better synchronization. EVA-01 is NERV's main combat unit and is painted dark purple and neon green.
EVA-02's parts were manufactured in Japan, but assembled and activated in Germany where the pilot Asuka Langley trained with the unit. It was brought back to Japan to work with EVA-01 and 00. EVA-02 has four main eyes, protected by armored shutters. This EVA unit is painted red and yellow.
Neon Genisis Evangelion