Android

Androids (人造人間, Jinzōningen; lit. "Artificial Human") are robotic/cyborg humans, most of which were created by the evil scientist Dr. Gero. Most of the androids are said to have unlimited energy and eternal life. Due to their inorganic nature, they, or at least the ones created by Dr. Gero, also have no detectable aura (unlike Saiyans, Humans, and others), so they cannot be directly tracked using Scouters or Ki Sense. There are at least 6 units shown to have been fully functional throughout the original Dragon Ball manga.

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Background

Dr. Kochin's Android form

Androids serve as main villains during the Android conflict, each serving a purpose of their own. Originally, all of the androids were plans used to kill Goku, however, most of the androids turn to the good side or are destroyed. However, in Future Trunks' Alternate Timeline, the androids succeed in their plans for world destruction, managing to kill all of the main Z Fighters (except for Goku, who dies of a heart virus, and Gohan, who survives but is killed 13 years later) until finally being destroyed by Trunks almost 20 years later.
There are three types of androids seen in the series, according to their power supplies:

1. The kind with no drawbacks and an unlimited power supply (such as #8, #16, #17 and #18). This type was said to be more difficult to control. They are immune to being directly detected via Ki Sense or Scouters due to lacking a ki-signature. However, they can be detected indirectly via their opponents' ki.
2. The kind which requires constant intake of energy, usually stolen from victims, to keep their power reservoir from draining, which would cause the android to malfunction (such as #19 and #20).^[1] The more energy they absorb from their victims, the stronger they get. Curiously, Dr. Gero oversees his configuration into the latter, rather than the type with no apparent limitations (though it is implied by #18 that this is because the energy-absorbing model is easier to control). This model, like the other Android models (exempting Bio-Androids), is immune to being directly detected by Ki Sense or Scouters due to lacking a ki-signature. However, they can be detected indirectly due to the depleting ki-signature of the victim.
3. The kind whose energy exists and circulates as a form of organic ki similar to that of naturally born beings, otherwise known as Bio-Androids (such as Cell and the Cell Juniors). These androids can be tracked by Ki Sense, though as Cell proves, they in turn can also learn to sense other's ki as well as mask their own ki signature to elude pursuit, just as organics can. In the case of Cell and all bio-androids based on him, their organic ki is derived from the cells of powerful humans, Saiyans, Namekians, and members of Frieza's race.

There are also three types of androids seen in the series, according to their "base" or materials:

1. "Cyborgs", androids made from an original human as a base (such as #8 (according to some guides), #13, #17, #18 and #20). Since they are human-based, they can become stronger if they train. Despite being referred to as "Androids", other than #20 they possess little to no robotic parts and the altercations to their being are simply a result of their organic matter being reconstructed.^[2] Though they do not need to eat, they do need to hydrate. Also, their cells deteriorate slowly, so they age slowly too.^[3]
2. "Total artificial construct types", androids which were made up completely from mechanical parts (such as #8 (according to Akira Toriyama),#14, #15, #16, #19, and Arale). For some reason, the totally artificial ones have a blood-like liquid crimson substance, presumably a kind of power oil.
3. "Bio-Androids", androids constructed either through biological engineering or as clones spawned from other bio-androids rather than from machine parts and/or a human base (Cell and the Cell Juniors).

Android avatars from Dragon Ball Heroes

As the most developed bio-android with the most versatile energy source, Cell also possess elements of the first two android power supply types as well. For one, he is able to absorb the energy of others and integrate them into his being through his tail, either by sucking them up or swallowing them whole. The latter method actually transforms him into a stronger state, and if he absorbs both #17 and #18 this way, he evolves into a highly potent form he describes as "perfection". Also, his cells can store memory of any form of power they ever taste, allowing him to return to that form if he should ever nearly become vaporized after losing it. Thanks to this and combined with the Zenkai healing factor of his Saiyan cell properties, he can in fact return from near death even stronger than ever before no matter how severely he should be weakened beforehand.

http://dragonball.wikia.com/wiki/Android