Astronuc said:
My university had several levels of professor: assistant p., associate p. and (full) professor.
Most places also have "instructors" who don't have PhD's. In some fields (esp. the humanities) it's not uncommon for someone to start out as an instructor while finishing his/her dissertation, then be promoted to asst. prof. and move onto the tenure track after getting that PhD. This is less common in physics, though.
Promotions seems to depend on teaching and research (and publication).
They can also depend on campus service (committees etc.) and community service. The weighting of these factors varies from school to school. At many small colleges, research and publication are relatively unimportant. Some require only a token amount of publication, some don't require it at all (although it's nice if you do publish).
Where I teach, for example, we're not specifically required to publish, but we
are required to show "professional development:" doing new things, engaging in professional activities, and maintaining outside contacts, so we don't get stale and burn out. Research and publication can be part of that, of course. As a church-related school, we're also big on community service.
I did see several faculty members more or languish as assistant or associate professor, so they moved on.
It's kind of hard to languish as an assistant prof, because most schools have an "up or out" policy. After a maximum of seven years as an assistant prof, you have to get tenure and promotion to associate prof, or leave.
At many schools, it's unusual for someone to be forced out after seven years, because there are ways of making the person realize that his/her chances of promotion are minimal. There are usually intermediate reviews by the department chair and the administration.
And at some of the very best schools (e.g. Princeton), it's unusual for an assistant prof to get tenure. Those schools usually bring in associate profs from outside, based on a proven track record of outstanding research. The ex-assistant profs usually move into associate prof positions at lower-level schools (e.g. the big state universities, or some of the better small liberal-arts colleges).