Each of these words identifies someone who communicates information or news. An informer, however, is one who informs (that is, tells on) someone else for the purpose of collecting money or other reward. An informer can be anyone who supplies facts or alleged facts: newspaper reporters and radio and television broadcasters are informers. An informant conveys to authorities information about criminal acts or other wrongdoing that is designed to lead to prosecution of the guilty person. The Internal Revenue Service, for example, relies on informants to supply information about persons who have not fully declared their incomes.
Dictionary of problem words and expressions. Harry Shaw. 1975.