Altruism, or effective altruism, is a social movement and philosophy that seeks to maximise the positive effects that individuals or organizations can make on the world. You use evidence and reasoning in order to identify the most efficient ways of benefiting others. Then you take action on this evidence.
On the other side, utilitarianism is a moral theory which states that the greatest action maximizes happiness and pleasure, while minimizing suffering or pain. According to Utilitarians, the consequences of any action determine its morality. The right action leads to greater happiness and satisfaction for all people.