Edit (1) 14/04/2013:
***End Edit (1)***Although jouissance can be translated as "enjoyment," translators of Lacan often leave it in French in order to render palpable its excessive, properly traumatic character: we are not dealing with simple pleasures, but with a violent intrusion that brings more pain than pleasure. This is how we usually perceive the Freudian superego, the cruel and sadistic ethical agency which bombards us with impossible demands and then gleefully observes our failure to meet them. No wonder, then, that Lacan posited an equation between jouissance and superego: to enjoy is not a matter of following one's spontaneous tendencies; it is rather something we do as a kind of weird and twisted ethical duty.
This simple, although unexpected, thesis encapsulates the way Lacan reads Freud. Freud uses three distinct terms for the agency that propels the subject to act ethically: he speaks of ideal ego (Idealich), ego-ideal (Ich-Ideal) and superego (Ueberich). He tends to identify these three terms: he often uses the expression Ichideal oder Idealich (Ego-Ideal or ideal ego), and the title of the chapter III of his booklet The Ego and the Id) is "Ego and Superego (Ego-Ideal)". Lacan introduces a precise distinction between these three terms: the "ideal ego" stands for the idealized self-image of the subject (the way I would like to be, I would like others to see me); the Ego-Ideal is the agency whose gaze I try to impress with my ego image, the big Other who watches over me and propels me to give my best, the ideal I try to follow and actualize; and the superego is this same agency in its revengeful, sadistic, punishing, aspect. The underlying structuring principle of these three terms is clearly Lacan's triad Imaginary-Symbolic-Real: ideal ego is imaginary, what Lacan calls the "small other," the idealized double-image of my ego; Ego-Ideal is symbolic, the point of my symbolic identification, the point in the big Other from which I observe (and judge) myself; superego is real, the cruel and insatiable agency which bombards me with impossible demands and which mocks my failed attempts to meet them, the agency in the eyes of which I am all the more guilty, the more I try to suppress my "sinful" strivings and meet its demands. The old cynical Stalinist motto about the accused at the show trials who professed their innocence ("the more they are innocent, the more they deserve to be shot") is superego at its purest.