An autobiography is the story of a person's life told by them and only them. A Long Way Gone is Ishmael's story and he's the one relating it all to us. We also can classify the book as a War Drama because the entire book takes place during a really awful war in Sierra Leone and it's also filled with more drama than anyone should ever have in his life.
Ishmael's book is really a memoir, which is really a subcategory of autobiography. There are some some interesting differences between autobiographies and memoirs that have some relevance to the controversy about whether Ishmael was really accurate in his descriptions of his experiences. A memoir isn't just an objective retelling of your life; it's your understanding of a particular time in your life and describes the emotional realities of that time. How people remember their experience and how it really went down aren't always identical.