They are two diffrent Martial arts. Tang soo do was first called Tang soo do, then the Korean government wanted to rename all Korean Martial arts Taekwondo. The founders/students of Tang soo do, and many other Korean Martial arts opposed this, so some of which actually sued the Korean government for the right to call it Tang soo do, and the rest is as they say history.
"There is no such thing as Perfection... Only excellence"