Journalists write the first rough draft of history.
Historians write the final draft, presumably.
Robert Caro, the biographer of Lyndon Johnson, thinks the main difference is time. The historian has the time to dig deeper and sift more thoroughly than the journalist can. Caro was talking about this the other night at the New York Public Library. Caro began getting it right only when Sam Houston Johnson, Lyndon's brother and the often drunken purveyor of family myth, sobered up and started talking straight to Caro. Sylvia Jukes Morris, now working on the second volume of her superb biography of Clare Boothe Luce, proceeded with the project for years before she discovered that Clare Luce had lied elaborately about her earlier life.