[phw]

You generally start as an assistant professor, which means you are on the tenure track but you don't yet have tenure. After several years, if you did well, you get promoted to associate professor, meaning that you got tenure. Finally, after some more years, you turn into a full professor, at which point you will likely earn the most.

All three types are generally referred to as "professor" but they differ in their seniority.

[impendia]

Promotion from associate professor to professor is not automatic; indeed, there are several associate professors in my department who have been at that rank for some time, and seem unlikely to get promoted.

But it is reasonably common, unlike in (say) the UK.