'Populists' (aka O'Reillyists) are those who go for the popular middle-class working-class votes using the same class division tactics used by the Democrats to woo the lower and lower-middle classes.
'Conservatives' are those to whom the Constitution means something, namely that the Imperial Federal Geovernment of the USA is not an income-redistribution scheme cooked up by the Founding Fathers.
The GOP does not usually live up the Conservative definition above, but hey you've got to get elected and the Constitution is a 'living document' anyway-- right?