Well... I don't know everything about how they work, but... Evangelion Unit 02 is part of a series of weapons made by an organization in Japan called NERV.
Fifteen years earlier, an Angel came to Earth and it initiated what's now called the Second Impact. I don't know why it came, or what it wanted, but it caused the destruction of most of the world. The Evangelions were created to prevent a Third Impact. Supposedly, they each contain a human soul and DNA. Unit 02... I realized it was made using the DNA of my mother. In order to pilot them, we sit in a plug of sorts that gets inserted into the Eva's back, and then filled with LCL, which is kinda like synthetic embryonic fluid. I know the Evas connect with us on a psychological level, so that any move we make, they make. It also causes any pain or damage felt by the Eva to be felt by the pilot.
Over the years, several more angels attacked, each more powerful than the next. Myself and two other pilots (they aren't important, I'm the best of them anyway) managed to defeat each of them.
For some reason, once we'd defeated the final Angel... we were attacked by what (to me) looked like mass-produced Eva units. The last thing I remember before arriving here was the mass Evas ripping Unit 02 apart, and me with it.