Original: It was written as a book "for children and those who love children" (as quoted from its subtitle) in 1880 by Swiss author Johanna Spyri. The two sequel books, Heidi Grows Up and Heidi's Children, were neither written nor endorsed by Spyri, but were written by her English translator, Charles Tritten, many years after she died.
Simplified: Heidi is a story written by Johanna Spyri published between 1880 and 1881.