In metonymy, a term is substituted for another term with which it is closely associated ("crown" or "sceptre" stands duty for "monarch"). In synechdoche, a part is used to signify the whole, as when a ship's captain calls out, "All hands on deck!" (in which "hand" signifies the whole person of each sailor--we hope).
In modern criticism, metonymy is often used in a more general sense, where some literature is seen as more concerned with relationships of contiguity rather than the kind of similarity that is explored in metaphor -- it is not because things are like each other but because they are next to each other in space or time. Metonymy is thus often seen as the controlling trope or figure for the loosely structured, open-ended works associated with post-modernism.