How do "lecturers" differ from "professors" in large universities? I notice that some universities have academic positions known as lecturers. Are these positions different from professors? Are transitions from instructors to the professor track relatively easy. This position seems similar to the adjunct professor position. 

Note that I am talking about universities in the USA.
I used to be a lecturer at a university in the southwest USA.  As a lecturer, I was charged with only teaching lower division undergraduate courses (freshman &amp; sophomore level).  In general, research and service is not a requirement for lecturers (unlike professors), but we are required to teach a larger load than professors.  And we were generally yearly appointments with no chance of tenure.