A monograph is a specialist book on a single subject written by one author. Monographs are usually intended to only be published one time.
(Think about the prefix mono, which means alone, single, or one.)
A serial can be a magazine or newspaper, which have many contributors and contain many different topics. A serial may be published weekly, monthly, or quarterly.
(Serial might remind you of the word series.)
Periodicals and journals may also be referred to as serials. However, journals usually refer to scholarly writing.
