Unlike the notion of illocutionary act, which describes the linguistic function of an utterance, a perlocutionary effect is in some sense external to the performance. It may be thought of, in a sense, as the effect of the locutionary act. Therefore, when examining perlocutionary acts, the effect on the hearer or reader is emphasized.
As an example, consider the following utterance: "By the way, I have a CD of Debussy; would you like to borrow it?" Its illocutionary function is an offer, while its intended perlocutionary effect might be to impress the listener, or to show a friendly attitude, or to encourage an interest in a particular type of music.