In the US, the hierarchy nomenclature goes:
Adjunct Professor (usually part-time, a grad student or a Ph.D.)
Assistant Professor (usually full-time, on a tenure track, but not always)
Associate Professor (usually tenured, but not always - some Ivies have non-tenured associates)
[full] Professor
named Professor (as in the blahdy-blady professor of blahdy blah)
hence, anyone who teaches in the US could be called a professor - it roughly means someone who is teaching a university class/
In England for the pre-1992 universities (the newer ones have different titles), the hierarchy is usually:
junior lecturer (sometimes divided between lecturer a and lecturer b)
senior lecturer
reader
Professsor
everyone below a professor is called "Dr." if they have a PhD. A professor is often considered as having a "chair" while the person running a department is called "head of department". In the US, a "chair" mainly refers to someone gaining a "named" professorship or becoming the head of the department.
The UK doesn't have a tenure system, so staff are on probation for a few years, but getting on and off probation doesn't correlate to the academic hierarchy. So a junior lecturer who gets off probation is still a junior lecturer.
Some universities are shifting to an American system, like Warwick recently did, making probationary staff "assistant professors" and post-probationary ones "associate professors"
But here's the bottom line (or as they'd say in the UK - at the end of the day - ), don' tget too worked up about what to call people. Pretty much anyone who isn't an a**hole, will tell you to call them by their first name nowadays. Even most "Professors" under the age of 50 also find it absurd to be called "professor".