What are EVAs, the Giant Battle Robots of 'Neon Genesis Evangelion'?
This post contains major plot spoilers for Neon Genesis Evangelion.
Neon Genesis Evangelion is one of the biggest and most influential anime series ever created, every episode shifting between philosophical introspection and giant-scale robot-vs-monster fights. The robots themselves, called Evangelions, or EVAs, appear as huge mechanical humanoids that must be piloted by a person inside a cockpit in the body of the robot, who "syncs" neurally with the EVA's brain to wield whatever weapons it has. Swords, guns, whatever. Seems pretty straightforward from here, but if you've done any googling about the show, or seen the first episode, or even finished the series and are still confused, you might be wondering: What IS an EVA, anyway?
I'm gonna start off by saying if you haven't seen the show yet and are planning to watch, DON'T go on the wiki and try to look stuff up, because you will be spoiled, and also confused. (That's nothing against wikis, they're very helpful, but be careful how you use them!!) Like any machine, there are rules of operation. The EVAs can't stray far from NERV Headquarters in Tokyo-3, as each is attached to a power source deep in the ground by an umbilical cord, which is plugged into the EVA's back and can generate its power indefinitely. Once the cord is unplugged or severed, the EVA has power for only a few minutes before it completely shuts down.
The EVAs' human pilots are shunted into them through an Entry Plug, where they are suspended in LCL fluid, a weird orange juice you can breathe like air. They also have to be piloted by certain people -- children who have been born after the Second Impact, and each EVA seems particularly attuned to only one specific person. Only Shinji can pilot Unit-01, only Rei can pilot Unit-00, and only Asuka can pilot Unit-02. Otherwise, the EVA's sync rates, shown in waveforms, will be completely mismatched.
If you've gotten far enough to see the conclusion of the second episode of the show, you've seen enough to know that the EVAs are not machines, not entirely. Shinji agrees to pilot EVA Unit-01, but, due to inexperience, fails miserably. Unit-01 is severely damaged by the Angel Sachiel; its arm is broken, and an eye is pierced before it loses power and Shinji faints from the phantom pain. Shinji can't remember anything, but in a flashback, we see the EVA come back to life on its own, without power, and viciously attack and destroy the Angel. Its damaged metal helmet falls off and Shinji watches it regenerate an eye -- a real, flesh and blood eye -- and he screams and passes out.
So, it's pretty clear that the EVAs are not all machine parts. The technicolor "armor" actually acts as a restraining prison for a giant humanoid body: the EVAs, under all that metal, are more similar to humans than they are to any robot. NERV created the EVAs to fight off the Angel threat, but the EVAs are more similar to Angels than machines, as each was grown from samples of the First Angel, Adam, after it was destroyed during the "Contact Experiment." (Each, except for Unit-01, which is the only EVA to be created using samples from the Second Angel Lilith.)
This is why EVAs can go "berserk" and move without being attached to an electrical power source: they're not machines at all, rather cyborgs, organic creatures with machinery grafted onto their bodies to keep their strength tamped down. Each EVA also contains a human soul: always the soul of the pilot's mother. Shinji's mother Yui died during the Contact Experiment, and her soul was fused into the body of the fleshy prototype Unit-01 when her body was destroyed. Asuka's EVA contains the maternal part of the soul of her mother, who went crazy as a result of her Contact Experiment and committed suicide. Rei's case is special since she doesn't have a mother, being a cloned fusion of Lillith and what was left of Yui Ikari, but with Unit-00's strange sync rates and tendency to go berserk or not work at all, it was never made entirely clear in the series if it was imbued with a soul or not. But the prevailing fan theory is that part of the first Rei's soul was salvaged and put into Unit-00, and the rest was used in Rei 2 -- hence, her aloofness around practically everyone but Gendo Ikari.
The Evangelion design was influenced by a Japanese demon called an Oni: designer Ikuto Yamashita said they were meant to have the look of "a giant just barely under the control of mankind." The look of them caused a stir, as they were a complete departure from the normal boxy, heavy design of the mechas in properties like Gundam -- toy companies were worried that they were too slinky and weird to be made into merchandise, but the sleek, sharp lines of the EVAs ended up becoming one of Evangelion's most recognizable characteristics.