we all know that Niall Ferguson is the Lawrence Tisch Professor of History at Harvard University, but what is the difference between his job and other professors of history at Harvard University? Where does this name of title come from?
Have you looked in the university's own website?
In general a university usually names a professorship after a significant scholar or benefactor. To know exactly what/why you need to get precise information from the department itself.... Or someone on here might know the back story on Lawrence Tisch.
>>What is the difference between his job and other professors of history at Harvard University?
In my experience, the main difference is that these "distinguished" professors make more money. And I suppose there is some prestige associated with an endowed chair (see the Wiktionary definition) as well.
Lawrence Tisch and/or his heirs set up a fund to 'endow'--finance, permanently-- a 'chair', a position. The person appointed to that chair (by some departmental committee) is going to get his salary fully (or in part) through these special funds; these supplement or replace the ordinary funding, to the holder's advantage, obviously.
The appointee's 'job' would be much like any other senior or notable professor, teaching and research, although the funding might allow him to shorten his teaching hours and lengthen his research committment. It could be that the person holding the chair is chosen because of his research interests, here, for example, a sub-field of history**. So the understanding would be that the endowment fund is paying for this appointee's research *in this particular sub-field.*
** e.g., modern economic history of Eastern Europe.
Here in Cambridge, as in Oxford, we have a number of “regius” (Latin for “royal”) professorships, endowed, generally a long time ago, by the crown (in many cases by Henry VIII). Only recently have universities in this country introduced the American practice of allowing non-royal plutocrats to purchase chairs named after themselves.