International differences between "adjunct professor" and "visiting professor"?
In the United States, "Adjunct Professor" is essentially an academic temp worker.  They are typically paid very poorly, have no job security, and often live near or below the poverty line.  They are often not well respected, particularly as many faculty perceive them as either a threat (administrators often use adjuncts to avoid making tenure-track appointments) or as a source of cheap labor.

A "Visiting Professor" on the other hand, typically means somebody who has a position elsewhere and is temporarily associated with the faculty, e.g., while on sabbatical or as a courtesy appointment for a person from industry who is teaching a course.  A visiting professor might or might not draw any salary from the university, depending on the particulars of their appointment.  Their status and degree of respect generally depend on their main position.

Note, however, that this vocabulary is not legally protected, and so some US institutions may change these titles around, particularly if they are seeking to camouflage the nature of an poorly paid post.