INSTRUCTOR: you need a master's degree, can't have graduate student, can only teach, empployment depend on the contract, contract renewed every year.
Assistant professor: entry level professor, need a doctoral degree, can persue research, can have graduate student, does not have tenure.
Associate professor: have all the power of assistant professor plus tenure, which means he can not be fired, unless he do something unethical.
Full Professor: have the same power as associated professor, need to be accomplished in a certain area of research.