Syntactic movement is a fact that must be expressed somehow by every grammar of human languages and was first captured by structuralist linguists who called it "discontinuous constituents"; other terms are "displacement", or simply "movement" (cf. Graffi 2001). It aims at capturing the fact that certain constituents appear to have been displaced from the position where they receive important features of interpretation (cf. Carnie 2006).
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A typical example is the following:
(1) which story has John told Peter that Mary knows of?
The phrase [which story] is interpreted as the object of knows of rather than the object of told, although it is closer to the latter verb. There can be different pieces of empirical evidence to support this intuition, based on pronoun interpretation or morphological case. For example, the possibility for a preverbal interrogative pronoun to be marked Accusative can only be explained if one assumes that it was moved from the postverbal object position, witness the ungrammaticality of (3):
(2) whom does he love?
(3)²whom loves her?
Within generative grammar it has been traditionally proposed that there are two instances of movement. Argumental movement (A-movement) displacing a phrase in a position where a fixed grammatical function is assigned, such as in movement of the object to subject position in passives:
(4) the book was read
Non Argumental movement (A-bar movement) displacing a phrase in a position where a fixed grammatical function is not assigned, such as movement of the subject or the object to preverbal position in interrogatives:
(5) who do you think loves Mary?
(6) whom do you think Mary loves?
A different partition among types of movements is phrasal vs. head movement. In fact, not only a phrase can be displaced, but also the head of a phrase, such as for example in (7) where the helping verb will has been moved across the subject position:
(7) will John arrive tomorrow?
Since it was first proposed, the theory of syntactic movement yielded a new field of research aiming at providing the filters that block certain types of movement, also called locality theory. Examples of bad movement of the A-bar type are for example the following:
(8) who do you think that Mary visited Peter before calling?
Different types of locality theories are illustrated in Manzini (1992).
There can be different ways to represent movement. In transformational grammar it has been signalled by a "trace" t, since at least the 1970s proposal by Noam Chomsky (cf. Chomsky 1975); another possibility which is common now is to assume that movement is in fact a process of copying the same constituent in different positions, deleting the phonological features in all but one case (cf. Chomsky 1995).
As for the reasons for movement there are at least two different lines of thought. The standard one, stemming from Chomsky's own proposal (cf. Chomsky 1995), is that it is driven by morphological-interpretative reasons (essentially, the idea that certain words containing morphological features like the wh-feature must be paired in a local configuration with similar morphological features); alternatively, it has been proposed that movement is rather driven by purely structural reasons related to the necessity to linearize words in a string (Dynamic Antisymmetry) based on a weak version of the so called Antisymmetry theory (cf. Moro 2000).