Tae Soo Do was the name some major kwans (or schools) of post-Korean war South Korea agreed to call their martial art.
The name Taekwondo was dropped due to various controversies and Tae Soo Do was chosen to be the new name for their martial art. The Korea Tae Soo Do Association submitted its documented creation to the ministry of education on September 22, 1961.
Choi Hong Hi was unhappy with this change and succeeded in 1965 changing the name back to Taekwondo with the reformation of the Korea Taekwondo Association.
Tae Soo Do is also a style created by Joo Bang Lee, as part of his Hwa Rang Do system. It is unrelated to the "original" Tae Soo Do, created by the unified kwans.