Von Ranke was the founder of the empiricist (scientific) school of historical thought. Prior to von Ranke, there was no clear distinction between history and literature. He was probably the first person to emphasise that history was a factual inquiry into the past, rather than just retelling stories without knowledge of whether they were really true (eg. Herodotus).
He stressed:
- the separation of the historian from his/her subject, ie don't interpret the evidence by modern standards, but extract true information about the past. Look at the sources in terms of the values the people who wrote them had, not the values you have.
- most attention should be paid to primary sources (he mainly used reports of Vatican ambassadors) in order to determine a correct historical record
- the historian should be looking for truth, rather than entertainment value.
His most well-known quote is that history wants only to show "wie es eiglentich gewesen", what actually happened, rather than provide the reader with moral lessons or entertaining stories. (That passage is in the BOS book of readings if you have it).
Hope this helps...