A matter of degree?
Professors are full-time faculty members who engage in undergraduate and graduate teaching, mentoring, research, and service. Only faculty members who are professors are eligible for tenure. Retired professors are accorded the honorific "Professor Emeritus" (or "Emerita" if the individual is female.)
An Associate Professor is a mid-level professor, usually tenured. motion from assistant to associate professor, usually five-six years following the initial appointment. Applicants are evaluated based on their contributions to research, teaching, and administration. The relative weighting of these contributions differ by institution, with PhD-granting universities usually placing more emphasis on research and liberal arts colleges placing more emphasis on teaching.
An Assistant Professor is an introductory-level professor, a position that is generally taken after the individual earns a PhD and/or completes a post-doctoral fellowship. After 4–8 years, it is usual for an assistant professor either to be awarded tenure or to be dismissed from the university--based on his or her teaching, research, publications, and record of accomplishment and involvement in campus life. By the way, the term "assistant" does not mean individuals who hold this rank are assistants to full professors.
A Distinguished Professor /Chair is an honorary position in which a full professor's salary is increased by being tied to an endowment that is derived from the University or a private donor.
An Adjunct Professor is a non-tenure teacher--usually an expert in his or her field--who is paid on a per course or per class basis.
A Teaching Assistant is usually a graduate students who assists hsi or her academic supervisor in teaching courses.
An instructor or lecturer is a teacher (often wwho does not have a PhD) who teaches an undergrauate-level introductory course in a particular field.
Academic tenure is a status that guarantees its holder the right to academic freedom.
Tenure protects academics when they dissent from prevailing opinion, openly disagree with authorities, or spend time researching, writing about, or speaking on unfashionable topics.
In that sense academic tenure is akin to the lifetime tenure that protects judges in Canada from external pressure.
Without job security, the argument is that the scholarly community might favor “safe” lines of inquiry.