The term adjunct faculty is used in the United States, while the same staff is referred as 'visiting faculty' or even lecturers in other countries. The job is the same and that is to cover a course foa shorter period of time.
The faculty is hired in the US universities to cover a course for a semester or a year, and if the university or college wants it can have them for another term or otherwise they are released immediately.
Adjunct faculty is hired on a temporary basis without any opportunity for becoming a permanent member of the institution. They do not have a proper contract signed that can protect their services from termination at the time of expiry of their terms.
The faculty is hired to teach electives for which the regular staff of the university or college is not available or there is no specialized staff for a subject that has been elected by some students.
The faculty does not enjoy as many employment rights as the permanent faculty, and they can be relieved from the services any time. However, if the students are happy the way they have taught them, the university might consider them again.
Sometimes these faculties, which have specialized subject qualification or industrial experts, are hired to boost enrolment in a college or university,
Adjunct faculty is completely different from tenured faculty. The tenured faculty has a proper contractual agreement with university or college and has chances to become a permanent staff member of the institution.