Basically, memoir is a fancy word for a true story.
But it's not quite as simple as writing down the true events that have shaped your life. Most good memoirs have a theme and a purpose—they're about something particular. Shaka's memoir is about a big transformation in himself that occurs during his prison experience. In other words, the theme is how he grew into a different person and became free…from both prison and violent, angry tendencies.
The stories of his early life explain how he ended up in prison, the stories of his time in prison cluster around how he eventually transformed, and the stories of life after show how he acts as a new person because of that transformation. It all fits together. And if you look closely, Shaka has worked very hard to give it a structure that ties back to that transformation…making it a classic example of a memoir.