Surface Structure : the actual spoken sentence.
Deep Structure : underlying meaning of the sentence.
A single deep structure idea can be expressed in many different Surface structures :
Deep Structure : Boy kisses Girl
Surface structure : The boy kissed the girl.
The boy was kissing the girl. The girl was kissed by the boy.
Surface and Deep Structure
The deep structure gives the semantic component of a sentence, while the surface structure gives the proper phonological information to express that thought.