Functionalism is a reaction against the "formal" linguistics theories that began with Saussurean Structuralism in the early 1900s. While Structuralism let go of diachronic language study (language change over time), in the 1970s Functionalism reopened diachronic study as a means of discovering the answer to how language change fits language function: change according to use. Functionalists focus on all categories of linguistics including phonology and syntax and grammar--though they do not separate these parts from the functional whole--in order to explain questions in linguistics, like "cross linguistic similarities of structure" (DeLancey). Conversely, Structuralists make no attempt to explain linguistics, "letting the structure simply be" (DeLancey).
Structuralism, begun by Saussure, focuses on structural interconnections in synchronic context (language at a synchronous, specifically selected, moment in time). It is synchronic interconnectedness that obviates the need for diachronic linguistic knowledge (language change overtime). Structuralists focus on signification in which the sign is isolated from the referent under the contention that meaning resides in the sign rather than in the objective referent (Abrams as cited in McManus). Structuralism's dominant question is different from Functionalism's: Structuralists seek to know, in an immediate synchronic context, the "workings of language" (Abrams in McManus) as in signification and difference.