There are multiple 'professor' titles, but only one tenured faculty role. The tenured faculty is the holy grail, and everyone else is underpaid and overworked. [0]

You have instructor, assistant professor, associate professor, and professor. There's also such things as visiting professor (temporary postition) or professor emeritus (used to be a professor).

[0]. https://www.bu.edu/handbook/appointments-and-promotions/clas...