An adjunct is generally not required to participate in the administrative responsibilities at the institution often expected of other full-time professors, nor do they generally have research responsibilities. The pay for these positions is usually nominal, even though adjuncts typically hold a Ph.D., but most adjuncts also hold concurrent positions at several institutions or in industry.
Adjuncts provide flexibility to the faculty, acting as additional teaching resources to be called up as necessary, however, their teaching load is variable: classes can be transferred from adjuncts to full-time professors, classes with low enrollment can be summarily canceled and the teaching schedule from one semester to the next can be unpredictable (furthermore, if the university makes a good faith offer to an adjunct professor of teaching during the following semester (dependent on enrollment), the adjunct generally cannot file for unemployment during the break). In some cases, an adjunct may hold one of the standard ranks in another department, and be recognized with adjunct rank for making significant contributions to the department in question.