Language can be conceived of as an association between form and meaning. When we speak, there is clearly a sound side (which is studied in Phonology) and a meaning side (which is studied in Semantics).
In GB, this elementary fact about language is captured through saying that utterances and sentences have a phonological form (PF) and a logical form (LF). However, language seems to be more than that. It does not seem to be unreasonable to pose that all languages have something like a morphology (even though this is controversial), and it is certainly true that all languages have a syntax.
In the early 1980 a model was developed that suggests that sentences are represented differently (that is, they look differently) at different stages of their generation. The assumption is that there is a state that sentences start out with, and that they unfold piece by piece to be audibly and comprehensively uttered in the end. Different levels of representation were proposed, namely Deep Structure (DS) and Surface Structure (SS), in addition to LF and PF. All of these levels are related by a general rule, called Move which says: "Move anything anywhere". This rule sounds very general and broad and does not seem to be appropriate for something that is structured in a certain way like natural languages seem to be. Nevertheless, it makes sense, since it interacts with a number of restrictions and requirements to make sure that only well-formed sentences will be uttered in the end.
Because of its T-like shape the model is called "T-Model" or, if you turn it around for obvious reasons, "Y-Model":
Note that this model is supposed to capture the generation of one sentence at a time. But different modules kick in at different stages of the derivation of a sentence. Starting with DS, -roles are distributed and X-Bar-Theory makes sure that phrases project in a uniform fashion. Then, at SS, Case-theory makes sure that all structural cases are assigned so as not to violate the Case Filter. Note movement needs to take place between DS and SS, since the discharging of thematic roles is a very local matter whereas the assignment of Case can be a non-local matter (thinking, for example, of NP-movement?).