Generally, monograph refers to a scholarly book or other one-time publication that is usually complete in itself; a book or treatise on a specific subject or person in which the treatment is detailed, but not extensive in scope. Most monographs include bibliographies.
In most disciplines, the sciences especially, monographs are no longer the predominate publication format. Journal articles are currently much more popular. Some libraries, again especially those concerned with the sciences, shelve monographs separately from journals.
Monographic catalog records are different than those for serial publications (also called periodicals). However, monographs can be published in series. A sequence of books, such as a group of books in a series, can be known as "serial monographs".