While the original Japanese word used in the series to refer to the Angels is shito ( 使徒, disciple, apostle), as opposed to the typical translation of 'angel' as tenshi ( 天使, messenger of heaven), the English translation was one specified by Gainax.
At another point, it is stated that the Angels are merely different evolutionary paths that mankind could have taken. Using the Biblical story of the tree of the knowledge of good and evil, the Angels are described as having eaten the 'Fruit of Life', giving them immortality. While the first fifteen Angels are descendants of Adam, humans themselves descended from Lilith.
The first two Angels, Adam and Lilith, are the progenitors of the other sixteen. Even though they are composed of an altogether different form of matter ('characterized by both particulate and wave properties, like light') the Angels' genetic code bears a 99.89% similarity to humans. Though they all have very different forms and abilities, Angels are often depicted with several common elements, such as the projection of AT fields, an internal power source called S² engine, and an externalized core, the destruction of which is often the only way an Angel can be defeated.