Utilitarianism can be summed up as a kind of moral arithmetic. To give a more precise definition, it is the belief that there are definable good and bad outcomes to actions, and that the ethics of a situation can be gauged by considering the potential benefit and/or harm which could result. Utilitarianism does not uphold rigid, unbreakable rules, but looks for pragmatic solutions, where there is a net benefit and harm is minimized. Utilitarians typically do have one single moral absolute, usually pleasure or satisfaction of self-interest, which is intrinsically good and is the basis for morality.
 Classical Utilitarianism
The version of utilitarianism promoted by Mill and Bentham holds pleasure to be the only intrinsic good. Bentham summarized classical utilitarianism by stating that "nature has placed mankind under the governance of two sovereign masters, pain and pleasure." This is often criticized in modern times for three serious problems: there is no difference between "base" pleasure and more sophisticated or intellectual pleasure; equality is not taken into account; pleasure is too abstract to be measured accurately.
 Preference Utilitarianism
Preference utilitarianism is an alternative to classical utilitarianism which defines satisfaction of personal preference or interests as "good", with pleasure being only one of these interests. The most vocal proponents of this type of utilitarianism are R. M. Hare and Peter Singer. Preference utilitarianism avoids many of the issues with classical utilitarianism because it recognizes other forms of "good" which might otherwise be overlooked in the name of pleasure. For example, classical utilitarianism cannot explain why adultery is wrong if one's partner is not made to suffer by it. In preference utilitarianism, the preference of the adulterers wife or husband matters even if the frustration of that preference wouldn't increase pain or minimize pleasure.
 Act vs. rule utilitarianism
Act utilitarianism evaluates every individual act, declaring it "good" if it produces the most utility out of all possible acts in the circumstances. Critics charge, however, that we cannot go through life judging every single circumstance individually - we need easy to follow rules to guide our moral conduct. We often cannot predict the outcomes of our actions beforehand - so we need to follow uniform, easily-understandable rules. Thus, rule utilitarians take a different approach - they seek to establish moral rules, choosing moral rules based on which rules will produce the most utility out of all possible moral rules. However, having so chosen those rules, the rule utilitarian then judges individual acts, not by the amount of utility they produce individually, but on conformance to those rules.
A good example: Stealing generally has bad consequences so rule utilitarians view stealing as unethical in all scenarios, even those when doing so would have overall good consequences. An act utilitarian can justify stealing medicine to save a child's life because stealing has the best overall consequences in that specific scenario, even though it normally has overall bad consequences. Rule 'utilitarianism' is, actually, deontology.
 Problems and criticisms
Many criticisms have come from people who believe in absolute moral standards other than those proposed in utilitarian thought, for example religious ones. Similarly, the human rights movement asserts that there are some universal standards of what people are entitled to, whereas utilitarianism does not. Most forms of utilitarianism considers only the effects of actions, and places no judgement on a person's motivations and whether they have "good intentions".
Hypothetically, classical utilitarianism could be used to justify the enslavement of a small minority in order to bring greater benefit or pleasure to the majority, and hence a net gain in happiness, despite the suffering involved and the fact that this would seem morally dubious to most people. This is not something that utilitarians have proposed, and it is not clear how preference utilitarianism would be affected, but is one logical application of the theory, according to some critics. This is an example of how utilitarian theory could conflict with ideals of human rights.
Another practical complication is that, arguably, every cloud has a silver lining: all the great disasters of history have led to some positive repercussions, if only in actions being taken to prevent such disaster recurring. With this in mind, it is difficult to weigh the immediate harm that these disasters caused against the long-term benefits they have prompted. However, this problem is not specific to utilitarianism but is an issue with all philosophies that have primarily human values.
 Attempts to correct Utilitarianism
Although Utilitarianism seems very logical, it can lead to some questionable moral statements, and many see the need to correct it. Some argue that the philosophy lacks attention to the individual and have proposed Ethical egoism as an alternative. Others see equality as the problem and have proposed Prioritarianism and strict egalitarianism as alternatives. Yet others say that Utilitarianism doesn't have enough Goddidit, but that is probably just bullshit.