Sigmund Freud is famous and infamous for introducing a sexuality based drive theory and model of the psyche. His 1905 Three Essays on the Theory of Sexuality is one of first works introduced by Freud regarding this important aspect of his theory.
The first essay on the theory of sexuality regards sexual perversions and aberrations. Here Freud distinguishes sexual aim- a desire - and sexual object- with which one wishes to fulfill that desire. Sexual aberrations are cases in which sexual aim is directed at a certain unaccepted (now) sexual object such as children or animals. Freud thought that we all might have the potential for such dispositions but that a proper course of sexual development would lead our aims away from these abnormal objects and in the direction of acceptable ones. If not, well...
The second essay on the theory of sexuality deals with childhood sexuality and here Freud lays out is famous theory of the psychosexual development track. From the moment we are born we have sexual energy, libido, which is transformed in our early years which shapes its aims and objects. Freud sketches this phases that have physical centers, starting from the oral stage, through the anal stage, on to the phallic stage, the latency stage and finally to genital stage (see other summaries here for elaboration on Freud's psychosexual development theory).
The third and last essay on the theory of sexuality, "The Transformations of Puberty", ties the sexual development of the adolescent with the events of his early childhood which produce his adult sexuality.
Summaries of important works by Freud:Sigmund Freud - The Interpretation of Dreams
Sigmund Freud - Civilization and Its Discontents