The main difference between a guardian and a testamentary guardian is when their responsibilities begin. 

Guardian

A guardian has the same legal rights and responsibilities as a child's parents for their care, upbringing, and growth. Guardianship usually ends when the child turns 18.

Testamentary guardian

A testamentary guardian's responsibilities begin after the death of the parent who appointed them. They have the same rights and responsibilities as the living parent or guardian, but only after the death of the appointor.

A parent can appoint a testamentary guardian in their will or deed. The testamentary guardian automatically becomes the guardian if the parent dies and was also a guardian at the time of death. If the parent was not a guardian at the time of death, the testamentary guardian can apply to the court to be appointed.