Generative semantics

Generative semantics was a research program in theoretical linguistics which held that syntactic structures are computed on the basis of meanings rather than the other way around. Generative semantics developed out of transformational generative grammar in the mid-1960s, but stood opposition to it. The period in which the two research programs coexisted was marked by intense and often personal clashes now known as the linguistics wars. Its proponents included Haj Ross, Paul Postal, James McCawley, and George Lakoff, who dubbed themselves "The Four Horsemen of the Apocalypse".
Generative semantics is no longer practiced under that name, though many of its central ideas have blossomed in the cognitive linguistics tradition. It is also regarded as a key part of the intellectual heritage of head-driven phrase structure grammar (HPSG) and construction grammar, and some of its insights live on in mainstream generative grammar. Pieter Seuren has developed a "semantic syntax" which is very close in spirit to the original generative semantics framework, which he played a role in developing.
Interpretive semantics and generative semantics.
The controversy surrounding generative semantics stemmed in part from the competition between two fundamentally different approaches to semantics within transformational generative syntax. In the 1960s, work in the generative tradition assumed that semantics was "interpretive" in the sense that the meaning of a sentence was computed on the basis of its syntactic structure rather than the other way around. In these approaches, syntactic structures were generated by rules stated in terms of syntactic structure alone, with no reference to meaning. Once generated, these structures would serve as the input to a semantic computation which would output a denotation. This approach captured the relationship between syntactic and semantic patterns, while allowing the syntax to work independently of the semantics, as Chomsky and others had argued for on the basis of empirical observations such as the famous Colorless green ideas sleep furiously sentence.
The generative semantics framework took the opposite view, positing that syntactic structures are computed on the basis of meanings. In this approach, meanings were generated directly by the grammar as deep structures, and were subsequently transformed into recognizable sentences by transformations. This approach necessitated more complex underlying structures than those proposed by Chomsky, and thus more complex transformations. Despite this additional complexity, the approach was appealing in several respects. First, it offered a powerful mechanism for explaining synonymity. In his initial work in generative syntax, Chomsky motivated transformations using active/passive pairs such as "I hit John" and "John was hit by me", which have different surface forms despite their identical truth conditions. Generative semanticists wanted to account for "all" cases of synonymity in a similar fashion, which proved to be a challenge given the tools available at the time. Second, the theory had a pleasingly intuitive structure: the form of a sentence was quite literally "derived" from its meaning via transformations. To some, interpretive semantics seemed rather "clunky" and "ad hoc" in comparison. This was especially so before the development of trace theory.
Notes.
 There is little agreement concerning the question of whose idea generative semantics was. All of the people mentioned here have been credited with its invention (often by each other). 
 Strictly speaking, it was not the fact that active/passive pairs are "synonymous" that motivated the passive transformation, but the fact that active and passive verb forms have the same "selectional requirements". For example, the agent of the verb "kick" (i.e. the thing that's doing the kicking) must be animate whether it is the subject of the active verb (as in "John kicked the ball") or appears in a "by" phrase after the passive verb ("The ball was kicked by John").