The sentences ""the dog chased the cat"" and ""the cat was chased by the dog"" have deep structure and surface structure.

Deep structure refers to the meaning of a sentence. Surface structure refers to how a sentence is worded.

In this case, this sentences "the dog chased the cat"" and ""the cat was chased by the dog"" have the same deep structure (the dog chased the cat) but different surface structure because the second one is in passive voice to give emphasis to the cat (the cat was chased by the dog).