Postmodern history is a reaction to the idea that history is a linear, factual object, distant and detracted from people.
In the general sense, postmodernism is a new catch-phrase relating to any kind of text (in this case, history) that draws your attention to it being a construct. This means that a postmodern historian will try to study the more text-response relation rather than historiocracy (the "Renet" Historian - "just the facts, ma'am"). Essentially, "what happened" is not really important, because things happen all the time. Why should history be important? A postmodern historian says history is important because it impacts us as people, and groups, in the present.
Hence, with different people, and groups, in the present arguing over history, how can there be "history"? I mean, if we don't know what happened, how is this a story or narrative of our past? There is no conventional history as we know it, because any and every history one creates will be altered by every experience one has experienced up till the very moment one types, writes, or speaks the new proposed narrative.
It is, therefore, important to acknowledge that history is not a linear, precise art, but rather a detective story with blurry witnesses and fragmented ideas and evidence. Making a movie like Titanic would be impossible because Rose and Leo had different perspectives, because there were 1000 other people experiencing the same event, and because *shock* Rose is actually an actor!
This is all good and well in the narrative known as film, but there's really no difference when a historian writes a book on history. When Breasted wrote his Records of Egypt, he was being influenced by his education style, the fact that some of his work was rushed, and the fact that half the important texts were burried under sand.
A postmodernist admits that we must try to understand "what happened", but pretending that our narrative is "what happened" is futile an a flat out lie. Hence, you find a pomo historian's love of primary evidence, alternate narratives, and constant reminders that he or she does not really know what happened, but is eager to present his view.