Louis Menand reviews recently published diaries, reviews the rationales behind keeping a diary, and makes a distinction, in passing:
“Never discriminate, never omit” is one of the unstated rules of diary-keeping. The rule is perverse, because all writing is about control, and writing a diary is a way to control the day—to have, as it were, the last word. But diaries are composed under the fiction that the day is in control, that you are simply a passive recorder of circumstance, and so everything has to go in whether it mattered or not—as though deciding when it didn’t were somehow not your business. In a diary, the trivial and inconsequential—the “woke up, got out of bed, dragged a comb across my head” pieces—are not trivial and inconsequential at all; they are defining features of the genre. If it doesn’t contain a lot of dross, it’s not a diary. It’s something else—a journal, or a writer’s notebook, or a blog (blather is not the same as dross).