To quote Ritsuko, an Eva is a thing created by man in mans' own image. In more detail though, an Eva is a copy of the first Angel found in Antarctica; Adam, however, modifications had to be made to allow NERV to control it. Unlike the Angels, an Eva doesn't possess an S2 engine, therefore energy is supplied via an umbilical cable. Should this cable become severed, an Eva can operate on an internal battery for no more than five minutes due to the current limitations in battery power, after which the Eva should supposedly cease to function.
Designated number: 00
First sortie: Episode 06
EVA-00 is the first Eva to be constructed, and is piloted by Rei Ayanami; the first child. In order for the prototype to be a viable option for close combat against Angels, NERV upgrade the Eva shortly after the eighth Angel is destroyed. Unit 00 has also been known to run amok on a couple of occasions. It was initially thought to have been caused by mental instability from within the pilot, however, it is later determined that the instability originated from within the actual Eva. It's highly likely that Unit 00's core was in fact built from the soul of Rei Ayanami.
Model: Test type
Designated number: 01
First sortie: Episode 01
EVA-01 has undoubtedly seen far more action in its time than any of the other Evas under NERV's control, it is also extremely unpredictable. On several occasions now Unit 01 has moved without energy being supplied to it and is quoted by Ritsuko to be a berserker. Though it is because of this Eva's rogue behaviour that many of the Angels were actually successfully destroyed and its pilot, Shinji Ikari; the third child, still alive to tell the tale. Also, it would be safe to assume that Yui Ikari's soul exists within Unit 01's core as a result of her synchronisation accident, this would explain Shinji's frequent visions of his mother when in Unit 01's entry plug. Unit 01 is also different from the other Evas in that it wasn't built from Adam, but instead from Lilith. As Lilith is capable of instigating the third impact, it comes of no surprise that Gendo and SEELE value Unit 01 significantly more than the other Evangelions. Unit 01 is effectively a viable substitute for Lilith should the need arise.
Designated number: 02
First sortie: Episode 08
EVA-02 is the first Eva production model to be constructed, and is therefore a template for future Evangelions, at least it was deemed to be the case at the time of its construction. Unit 02 is piloted by Asuka Langley Soryu; the second child, and is the only Eva to have experienced extreme conditions such as engaging an Angel in aquatic combat, and worst still beneath seventeen hundred meters of molten lava. Unit 02's core was also built from the soul of Kyoko Zeppelin Souryu as a result of her synchronisation accident.
Designated number: 03
First sortie: Episode 18
EVA-03's existence is that of a very short one. It's first test activation at Matsushiro goes horribly wrong as the dormant thirteenth Angel within the Eva's body awakens taking control of the Eva, helplessly trapping its doomed pilot, Toji Suzuhara; the forth child, inside its entry plug. NERV classify the Eva as the thirteenth Angel and engage it with Unit 01 under the control of its dummy plug. Although the pilot is fortunate enough to survive Unit 01's attack, Unit 03 is completely dismembered and rendered useless after its bloody confrontation with the frenzied Eva.
Model: Experimental S2
Designated number: 04
First sortie: Episode 17
EVA-04's existence is even shorter than that of Unit 03's. Unit 04 is equipped with NERV's experimental S2 engine, which is similar to that of an Angel's. It works by converting matter into energy with no losses, but its initial activation at the second branch of NERV in the Nevada desert, America, goes horribly wrong. As a result NERV-02 is wiped off the map along with Unit 04, its newly installed experimental S2 engine and thousands of people. An explanation for the vast loss of life is not given, but it's probably safe to say, based on the fact that the instillation just vanished, that the engine's containment degraded, subsequently converting all mass within a set radius into pure energy.
Model: Mass production
Designated numbers: 05-13
First sortie: End of Evangelion
Not much is known about the other Evas, other than the fact that they were built at seven different locations around the world in a hurry, and that all these Evas include working S2 engines. They are also known to be capable of extremely rapid regeneration much like that displayed with Unit 01. As well as this, Units 05-13 are relentless in their attacks as none are piloted by a human, instead each is piloted by a dummy plug and so morality isn't an issue for them when engaged in combat. And as if this weren't enough, Units 05-13 are all flight capable, in fact they could all be very well likened to vultures. They are vicious deadly creatures who wouldn't think twice before tearing apart their prey.