Utilitarianism is often used in conjunction with the term “consequentialism”. This is the belief that all that determines an action’s morality is action’s consequences. Utilitarian’s also believe that a “moral” act increases happiness the most, for the most number of people.  There are two main theories of Utilitarianism: act-Utilitarianism and rule-Utilitarianism.
Act-Utilitarianism states that something which is moral is merely an act which benefits the most number the most amount. So, under this system, it would be moral to kill the hiker in the famous “Transplant Problem” (related to the Trolley Problem). 
A common criticism of act-Utilitarianism is that it allows for, and in many cases even openly encourages, the murder of individuals for the “greater good”. This is something that many people dislike, and as shown by most people not killing the hiker in the transplant problem, most people do not subscribe to a system of pure act-Utilitarianism.
In addition, happiness is not something easily calculated. Claims of happiness are completely unreliable and are likely to be exaggerated. Few are also hooked up to a machine which measures the amount of emitted Dopamine in their brains.
Contrast this with rule-Utilitarianism, which states that an action which is moral merely follows maxims that are Utilitarian. In the transplant problem, the moral thing to do would be to not kill the hiker, because a society where innocent passerby could be killed for organ harvesting would likely not be a society which produces happiness in the long term.
In this case however, again, happiness is not something which can be measured. In addition, long-term rules also suffer from lack of foresight to the future. For instance, within the transplant problem, it can be argued that, “A society in which a (comparatively) large majority of people may be left to die will likely not produce the greatest amount of happiness.” Furthermore, if the patient in need of organs were to experience more pain than the killed would, it could also be argued that, “A society which allows a comparative majority of people to suffer more pain will likely not produce the greatest amount of happiness.” In each of these cases, rule-Utilitarianism appears to lead to two contradictory conclusions.
Overall though, a long-term form of Utilitarianism appears to me a far more realistic form of ethics, and one that encourages the murder of innocents far less.
The form of Utilitarianism I find to be most effective is John Rawl’s “Veil of Ignorance”. Essentially, the Veil of Ignorance dictates that an action made under a “veil of ignorance”, a form of objectivity, is the moral action. This veil of ignorance encompasses any personal characteristics, such as wealth, race, gender, sexual orientation, but also less commonly thought of aspects, including tendency to laziness, intelligence…etc.
For instance, under this system, few people would create a society with slavery because they wouldn’t risk being a slave themselves, therefore slavery would be immoral.
Criticisms have arisen however, over the elimination of personal bias. Ayn Rand argues that it is impossible to make a decision based on ignorance, which seems strange, given that full knowledge is impossible. Many also note that the Veil of Ignorance does not adequately address personal beliefs. For instance, a more risk-loving individual may choose to instead take the gamble of being filthy rich or shockingly poor, while a risk-averse person may choose to instead minimize their losses.