Utilitarianism by John Stuart Mill  is a classic exposition and defence of utilitarianism in ethics. The essay first appeared as a series of three articles published in 
Fraser’s Magazine in 1861 (vol. 64, p. 391–406, 525–534, 659–673); the articles were collected and reprinted as a single book in 1863.
Mill’s aim in the book is to explain what utilitarianism is, to show why it is the best theory of ethics, and to defend it against a wide range of criticisms and misunderstandings.
Though heavily criticized both in Mill’s lifetime and in the years since,  
Utilitarianism did a great deal to popularize utilitarian ethics and has been considered ‘the most influential philosophical articulation of a liberal humanistic morality that was produced in the nineteenth century.
Mill took many elements of his version of utilitarianism from Jeremy Bentham, the great nineteenth-century legal reformer, who along with William Paley were the two most influential English utilitarians prior to Mill. Like Bentham, Mill believed that happiness (or pleasure, which both Bentham and Mill equated with happiness) was the only thing humans do and should desire for its own sake.
Since happiness is the only intrinsic good, and since more happiness is preferable to less, the goal of the ethical life is to maximize happiness.