The standard Evangelion design is based on the that of the original Angel, Adam.
The are humanoid in appearance, but are coated in a thick layer of armor which "bonds the Eva to the will" of the humans. Inside each Evangelion is a human soul, an instrument used by NERV to further humanise these terrifying weapons. Each Evangelion has a space at the back of the neck reserved for the Pilot's Entry Plug. This area acts as the Evangelion's cockpit and provides full control for the pilot. Inside this cockpit is the liquid known as LCL, which allows the pilot to better synchronise with the Evangelion. The LCL also directly oxygenates the pilot's blood, which is how they can breathe inside the Entry Plug, regardless of situation.
Despite their robotic exterior, Evangelions appear to be at least somewhat, if not entirely, organic. Sometimes, the Evangelions have even shown a human-looking mouth with teeth, though this only occurs during a Berserker transformation. The armour used on on an Evangelion is considerably stronger than anything else used by the military, as it was capable of withstanding the self-destruction of the Third Angel, Sachiel, which had itself shrugged off attacks by N2 mines, which are able to decimate entire cities. It is worth noting however, that this level of durability may have been a result of Evangelion Unit 01's unique Berserker mode.
Additionally, all Evangelion Units are equipped with their own personal AT Field (also known as the Absolute Terror Field). The Evangelions are capable of using this AT Field to shield themselves from attack, disrupt attacks of others and neutralise the AT Fields of Angels.
Evangelions rely on a corded power supply with various outlets located throughout out Tokyo-3 in order to remain functional. These power outlets allow the EVA to draw energy from NERV headquarters. However, if a Unit's cord is severed, the EVA is not rendered completely useless. Instead, they are able to rely on back up power that can last between one to five minutes, depending on the strenuousness of the activity in which the EVA is engaging. Should an Evangelion remain completely immobile, with all vital systems save life support shut down, the Unit can last for much longer; approximately twenty-four hours; as was the case when pilot Shinji Ikari was sealed inside the Sea of Duroc while combating the Twelfth Angel; Leliel.
Evangelion Unit 01 was unique amongst the Evangelion series both for its tendency to go Berserk, and the inclusion of an S2 Engine (which it gained from consuming the remains of a defeated Angel), allowing the Unit to continue without an external power source.
In addition to using the AT Field as a weapon, the standard Evangelion Unit is equipped with a large assault rifle known as a Pallet Gun, and a tactical knife known as the Progressive Knife; an extremely advanced weapon capable of piercing AT Fields, and, with sufficient force, the Core of an Angel. The Progressive Knife is capable of severing matter on a molecular level, hence why it is able to cause such devastation to normally virtually invulnerable objects.
In addition to the above weaponry, Evangelions have also been known to use hand guns, beam rifles, sniper rifles, spears and halberds.
In order to defeat the Fifth Angel, Ramiel, Evangelion Unit 01 was equipped with a Positron Sniper Rifle; a highly advanced rifle that used the energy from the entire city of Tokyo-3 in order to power it. The rifle can only be used from a laying down, prone position. Consequently, this leaves the user highly open to counter-attack. In the attack on Ramiel, NERV compensated for this weakness by modifying a decommissioned military jet into a large shield, and having Eva Unit 0 defend the prone Eva Unit 01. Aside from the Evangelion itself, the Lance of Longinus and the N2 Mine, it is possibly the most powerful weapon in the NERV arsenal.
On rare occasions, when the pilot of an Evangelion is either enraged or endangered, the human soul within the Evangelion is capable of unlocking its "Berserk" form. Once initiating its Berserker aspects, the Evangelion takes on a far more animalistic stance, gains advanced regenerative powers, increased strength, and the ability to withstand ordinarily fatal attacks. Perhaps most importantly though, while Berserk, the Evangelion does not require an outside power source.
Additionally, Unit 01 was capable of absorbing the Fourteenth Angel's S2 Engine, allowing it to function without an outside source permanently, which was a requirement for SEELE to initiate Third Impact.
The down side of the Berserker form is that the pilot cannot control the Evangelion, and both friend and foe can be targeted. Upon activating the Berserk form of Evangelion Unit 01 while fighting the Fourteenth Angel, Zeruel, the pilot, Shinji Ikari, was assimilated by the LCL inside the cockpit of the Eva. Shinji's physical form was disseminated throughout the LCL, and was only restored after an intense psychological ordeal. This ordeal largely consisted of Shinji having to separate his consciousness from that of the Eva's, and create his own distinct existence which was not reliant upon that of his serving as an Evangelion pilot. No other pilot has reached this level of Synchronicity with their Eva, meaning that the it is impossible to know whether the ordeal would change depending on the Pilot's psyche.
Shinji and Evangelion Unit 01 are not the only Pilot and Eva to have achieved Berserker mode, however. After being defeated by the Mass-Produced Evas, Asuka and Eva Unit 02 achieved Berserker mode for a moment, after Asuka realised that it was in fact her mother's soul that was infused within her Evangelion. The Eva's Berserker mode was considerably less powerful than Unit 01's, though, as Unit 02 was only able to raise its arm.