The concept of the new children or Indigo children or star children was first introduced by Kryon, Lee Carroll and Jan Tober. There have been many definitions of what Indigo children are. Usually, they are known as very old and highly evolved souls who have incarnated back on earth to help in raising the vibrations of the earth plane to assist in the shift in human consciousness.

As humans have a tendency to compartmentalize everything, some have labeled indigos as having certain traits such as the tendencies to have Attention Deficit/Hyperactivity Disorder, are rebellious, system busters, highly intuitive, empathic and intelligent, etc. Indigos are also thought to have a different set of DNA that is more evolved, and also has a strong remembrance of their connection with spirit.

Common views are that a handful of Indigos have incarnated since the earlier part of the last century with an Indigo overlay, with the majority and pure Indigos coming in from the 70s onwards. It is also believed that even more highly evolved souls have incarnated as crystal children from the 90s followed by the rainbow children in the 21st century.

Further reading

Kryon, Lee Carroll and Jan Tober Indigo site
http://www.indigochild.com

Doreen Virtue: Indigo and Crystal Children
http://thecrystalchildren.com/crystal.html

Wendy Chapman on Indigo
http://www.metagifted.org/topics/metagifted/indigo

Carla Rueckert: Indigo Children are a Mystic Handful
http://www.cosmiclighthouse.com/dec07/columns/carla_rueckert
 

Interview with an Indigo
http://www.cosmiclighthouse.com/aug08/articles/interview_with_an_indigo