An instructor is someone who lectures in front of a class. The job classification, status, and background of such people varies widely. Here is a brief guide to the types of people you might encounter as instructors.
The honourific "Professor" is usually applied to an instructor in the classroom, but the term is more properly applied to someone holding the rank of Assistant Professor, Associate Professor, or just plain Professor (which implies "Full Professor"). These ranks are usually held by research faculty, who carry out research and supervise graduate students as a significant part of their job.
An Assistant Professor is usually hired on a three-year probationary contract, renewable once. These are people who have a PhD and a research record. At the end of the second contract, they must leave the university if they have not successfully completed the promotion and tenure process, which involves a committee asking for arm's-length evaluations. If they are successful, they are promoted to Associate Professor and given tenure, which means that they can only lose their job for severe dereliction of duty. Promotion to Professor is not automatic; it may occur after several years, and many professors retire never having achieved the rank. People coming from other institutions may start at the Associate or Full rank, depending on their history.
Typically, a professor is expected to spend 40% of their time on research, 40% on teaching, and 20% on committee service. Those proportions will vary depending on the individual; for example, someone with a senior administrative post (Chair or Director) will have a higher service component. Usually, these administrative posts are filled on a rotating basis (for example, a chair or director may serve for two to five years). While teaching is taken into account in the tenure and promotion process, it is primarily research that counts. A poor research record cannot be salvaged by good teaching; a good research record will occasionally be torpedoed by bad teaching.
The professors, together with lecturers, make up the official faculty of the University. The job of a lecturer is primarily undergraduate teaching. They may also support the undergraduate program in other ways, for instance through student advising. Lecturers have a higher teaching load than research faculty, and tend to teach first- and second-year courses. Some lecturers are on two- to five-year contracts; some have a status similar to tenure.
Since lecturers are devoted to teaching, you might think that they would do a better job than professors with their divided loyalties. But this is not always the case. As with professors, some lecturers are excellent; some are mediocre. Two things that might limit the effectiveness of lecturers are the higher teaching load and the lack of connection to current trends in the field.
Other instructors are called sessionals, and they come from a variety of sources. Some are visiting professors, on sabbatical leave from other institutions; some are postdoctoral fellows, who have finished a PhD but have not yet obtained a professorial position; some are academic support staff who might do some teaching as part of their duties; some are graduate students; and some are people with experience in the field. Some sessionals teach only once or for a year; some are hired on a regular basis. The university tries to limit the number of sessionals, because such people may not have a complete picture of the curriculum and where the course they are teaching fits in, and it is difficult to ensure quality and continuity. But, as with the other categories, a sessional teacher may be excellent or mediocre.
Whatever the classification of your instructor, they have a responsibility to do their best in the classroom. Understanding a little about their situation may help you to interact better with them. --PR