Usage may vary, sorry. The following are only my rough impressions.
Academic rankings are (top down) professor, associate professor, assistant professor, and lecturer/instructor. Professors have tenure (guaranteed lifetime employment), and associates/assistants are tenure-track (eligible to become professors). Lecturers/instructors may be full-time employees, but they're usually not eligible for tenure, so their contracts can be canceled.
'Visiting' can be added to any of the above, depending on the level of prestige. At the higher levels it usually means that the person is either on the faculty of another university, perhaps on sabbatical there, or is accomplished in a nonacademic field (writer, artist, etc.)
'Adjunct' is generally less honorary, often someone paid by the course or by the hour (as opposed to getting a salary), on a temporary or contract basis (even if for an indefinite period). It can now be a noun as well as an adjective.
See for examplehttp://www.aaup.org/statements/SpchState/cont...http://www.aaup.org/publications/Academe/2004...