the purpose of memoirs is usually to recount something from your point of view so that it's in the public record. Historians might make use of memoirs published by different generals, for instance, in writing war histories, because it gives them a range of points of view.
Autobiography is usually a much bigger project telling the whole of somebody's life - like a biography, only written in the first person - and often including a degree of self-analysis. But some books that get called 'memoirs' are not much different from this; a general's memoirs may actually recount his whole life. So there's no hard-and-fast distinction.