I would agree with what the others have said. Rank is a reflection of how long you've been around and what you've contributed to the department, hospital, medical school, and field. The ease of advancement depends on the level of prestige of the department and what your niche is. Many places, including my own, have tracks, either formally or informally, and your accomplishments/contributions are evaluated in light of your track. We have 3, basically: researcher, educator, clinical/quality/admin. The idea is that you basically pick one in your first year or two and then work towards excellence or acknowledgement in that area so that by year 6 you have something to show the promotion committee. Evaluation for promotion typically included evaluation within the department and medical school, but also there's often an external review (by faculty at other, peer institutions). Sometimes this comes in the form of letters of rec from outside faculty.
The terms of advancement are often, as they are here, laid out pretty explicitly by the medical school.
Advancement brings with it more money and "prestige," whatever that is. At my department, the monetary increase is modest and amounts to about the same increase as the annual increases within a given rank, so it's not a huge difference in pay. I haven't noticed that promotion = less work (aside from the fact that qualifying for promotion requires accomplishing the kinds of things that ALSO qualify a person for more non-clinical time to actually DO those things). That is, it's not like you jump to associate and get handed an extra free day, but rather to GET to associate you have to achieve a bunch of things for which the department would award you non-clinical time.