It would be very difficult to summarize the influence of Marx in less than a very thick book. To put it simply, his writings have had a very strong influence on most branches of the humanities. There are Marxist historians, post-Marxist historians, Marxist sociologists, etc. The list is long.

This does not mean that folks who study along Marx' lines of thought are advocating some sort of communist revolution. What it does mean is that lots of academics have chosen to see the past (or their area of study) thru at least one of the lenses that Marx posited. His most famous, that "The history of all hitherto existing society is the history of class struggles" is likely the most influential. At its most simple, Marx means that historical change has been driven by economic differences (rich/poor) and the struggle by the wealthy to maintain that difference in the face of the struggles of the poor to overcome the difference.

While Marx was most definitely speaking of the class struggle between the wealthy and the poor economic classes, many scholars have expanded the definition of 'class' to show that the dialectic struggle Marx (and Engels) described can be applied to many different lines of inquiry. It's easy to see the struggle between rich and poor. It may not be obvious to those who don't study it, but there are also dialectics between the educated and the uneducated, between heteronormativity and homosexual acceptance, between atheists and religious folks, between paternalism and feminism, between thin people and fat people, between whites and people of color, etc. All of these groups can be seen as a 'class' whose interests are in conflict with the interests of the opposing class.

It's important to remember that a 'Marxist historian' is not necessarily a 'Marxist.' It's entirely possible (tho admittedly unlikely,) to be a very staunch political conservative and still be a historian who sees the past in terms of class struggle.