The Nature of EVAs
Neon Genesis Evangelion's EVAs are a core part of the series, and their unique nature means that they also have a major role to play in the plot.
EVA Unit-01 in action
As a mecha anime series, Evangelions are biomechanical cyborgs rather than robots--essentially giant humans wearing armor that can be controlled. This is shown several times when the armor is damaged, and human hands complete with fingernails are revealed underneath.
Evangelion Unit 00
Each EVA has within it a human soul as a result, and the EVAs were designed based on Adam, the First Angel, meaning they are connected to the very Angels that they're supposed to fight.
Neon Genesis Evangelion Eva Unit 01
The armor that covers the EVA isn't just for defense but is also a restraint placed on the EVAs to keep them under control and to keep the public from finding out they're not just robots. Unlike most mecha, Evangelion's EVAs can only operate independently off of a battery for a brief period and are usually shown fighting while connected to an external power source via an umbilical cable to the back.
The EVA Units
In the Neon Genesis Evangelion series, each EVA unit is fairly unique, with its own problems and capabilities, and can essentially only be piloted by a single specific person.
Evangelion Unit 01
Here's a rundown of the EVA units seen throughout the series, what they can do, and what's known about how they were made.
Evangelion Unit 02
Detailed Analysis of Each EVA Unit
Unit 00, also known as the Prototype unit, is an (at first) orange EVA piloted by Rei Ayanami. It was the first action-ready Evangelion constructed, and as such has some very different capabilities from the others. It's unable to use certain equipment, such as the Magma Diver suit, making it less useful overall than the other units.
Rebuild of Evangelion's version of Unit 01
Unit 01, sometimes called the Test-type Unit, is the purple and green EVA piloted by Evangelion's protagonist, Shinji Ikari. It seems to be the most powerful of the EVA units, as well as the most difficult to control. Unit 01 even devours the Fourteenth Angel, Zeruel, which results in a significant power upgrade.
Evangelion's Unit 01 going berserk against Zeruel.
Unit 02, the Production Model EVA, is the red and orange EVA piloted by Asuka Langley Soryu. Like Unit 00, Unit 02 is based on Adam. It was specially designed to fix problems experienced with its predecessors and is overall a more reliable EVA, although its pilot isn't nearly as reliable.
Evangelion Unit 03
Unit 03 and 04 are twin units that were built in the US. Unit 04 is destroyed in a failed experiment, forcing Unit 03 to be shipped to Japan. The Thirteenth Angel Bardiel infects the EVA, and a berserk Unit 01 ends up ripping the Angel-Unit 03 hybrid to shreds, killing the enemy while severely injuring the pilot trapped inside.
Evangelion Mass Production Eva
The Mass Production EVAs are very different from the other Units, both in appearance and capability. They do not look nearly as humanoid, with long snout-like, eyeless faces and no clear necks. Each Mass Production EVA has a set of wings enabling it to fly, and they aren't restricted by the need for an umbilical cable or even a pilot. The Mass Production EVAs are unleashed by SEELE to destroy the NERV headquarters, and Asuka in Unit 02 fights them off, but ultimately loses.