Sentence:  The incorporation of a rule of V <small>MOVEMENT</small> into our description of English <WSD>Syntax</WSD> turns out to have fundamental theoretical implications for our overall Theory of Grammar: it means that we are no longer able to posit that the syntactic structure of a sentence can be described in terms of a single Phrase-marker representing its S-structure. For, the postulation of a rule of V-<small>MOVEMENT</small> means that we must recognise at least two different levels of structure in our Theory of Grammar — namely, a level of "D-structure" (formerly known as ‘Deep Structureʼ) which serves as input to the rule, and a separate level of "S-structure" which is formed by application of the rule. 
Meaning of the word marked with <WSD> tags:  A set of rules that govern how words are combined to form phrases and sentences.