A memoir is based on real-life experiences of the author, while fiction is made-up stories that don't have to be true. Memoirs are often more personal and reflective, while fiction gives the author more freedom to create characters and plotlines.
The main difference is that memoir is a factual account of someone's life or a particular period in their life. Fiction, on the other hand, is entirely imagined by the author. In memoir, events and people are presented as they actually happened, but in fiction, anything can be invented for the sake of the story.
Memoir is rooted in real events and emotions the author has lived through. Fiction is all about creativity and imagination. With memoir, you get an authentic look into someone's life, but fiction offers endless possibilities and fantasy.