Memoirs is a French word which literally means 'memories'.
Memoirs, however, aren't biographies by definition. They're often selected chapters from the real life of the author. 'Biographical memoirs' are a class of memoir in their own right.
Memoirs can deal with a special event, a period in the life of the author, or other specialist subjects. They also frequently contain the author's opinions and analyses of the subjects.
Memoirs have another relevance, not always found in biographies. They form part of the historical records of their subjects. It's not uncommon, for example, for personal memoirs from combatants on both sides to form part of a military history.
There are even fictional memoirs, like The Memoirs of Sherlock Holmes, a series of episodes in the life of the famous detective.
How to write memoirs
Memoirs are written in book or short story formats. The basic narrative outline is a specific time or subject. The general approach is historical, from the writer's personal perspective.
The major point of interest in memoirs are the big events, or historical episodes. This is non fiction, and unlike fiction, there's a strong requirement for factual information.
Because memoirs are personal, the choice of a starting point is up to the writer's discretion. From then on, however, the narrative has to follow the historical sequence of events, to a large degree.
The overall structure of memoirs is fairly simple, but the organization of the memoirs into chapters and coverage of events is extremely important:
The introduction in memoirs is frequently an important part of the book. It's often a personal note to the reader, expressing the views of the writer about the material. It adds further perspective to the material. In many memoirs, it's also often written sometime after publication, as additional information.
This is where the initial historical sequence of the memoirs is created. It explains the author's position in the memoirs, and gives the scene for the storyline. Generally this is a purely biographical account, to orient the reader to the story.
(Note: Many memoirs are written very much as histories, complete with bibliographies, and fully documented. Some memoirs really are historical works in their own right.)
Development/explanation of sequence of events
This is usually the start of the main story, which may be divided up into chapters or episodes of the author's life. It also leads up to the main scenes and themes of the memoirs, in which the author plays a role.
This is also where the organization of memoirs becomes critically important. Even when writing in a purely historical sequence, there are always concurrent events which have to be introduced. Often, they're used to introduce major elements in the story, like 'I was working there when the war broke out…', or 'This was where I first met John F Kennedy'.
Each of these elements creates the picture for the reader. It's necessary to keep the memoirs in context not only with the author's role, but with the bigger picture, to understand the main events in a time line.
This series of events is the heart of the memoirs. For the sake of clarity, the events are normally broken up into a series of related chapters, so these events can be covered in detail.
The memoirs are now full historical narratives, and they have to cover both personal and big picture perspectives. Memoirs of the Great Depression, for example, are often set as personal narratives with an ongoing coverage of the events in the wider perspective as a backdrop. Memoirs written during wars often work in the opposite sense, where memoirs are written with the author of the memoirs as a player in the big picture. Political memoirs are often written using both styles.
Memoirs are, by definition, first hand accounts of their times. In some cases they form the only historical record of some events. Whatever the style, the memoirs also include a lot of firsthand information, which is invaluable to historians, as well as other readers.
After the main events chapters, memoirs wind down, with a 'looking back' perspective, and some information about events afterwards. The normal format is a synopsis of the subsequent life of the author.
Commentary in memoirs
Memoirs have always been part of history, in some form. Another, very relevant, part of the process of writing memoirs is the fact that most include direct personal commentary on the events they cover. Caesar's Commentaries was one of the first best selling memoirs.
The modern personal perspective in history was pioneered by Cornelius Ryan in The Longest Day, but it's been around since Caesar in biographical and other forms of literature. Caesar's motives in his commentaries, like many modern politicians in their memoirs, were purely political. Memoirs are more than history; they're personal views, expressed by the people who lived through those events. They add a unique perspective which isn't possible in other histories.
Personal commentary in memoirs adds other information, too. Not all memoirs are accurate. Many are self serving, or simply evasive. The first known memoirs were written by a scribe writing on behalf of Egyptian pharaoh, Thutmose III, circa 1479 BC. They were also one of the first examples of propaganda in history, with the pharaoh claiming to have personally attacked the Syrian enemy in his chariot at the battle of Meggido. Subsequent archaeology has since proved this to be entirely incorrect.
Memoirs aren't written under oath. They're the product of the author's knowledge and personal interests. That alone makes them true history, when seen against other perspectives, and that's their real value.
Examples of memoirs:
These examples are free online.