Effective Altruism is a research field, philosophy, and social movement. It is about using evidence and reason to to find and pursue the best ways to improve the world. It offers an approach that makes different social challenges comparable, helps prioritising between them, and supports people in taking action in ways that fit them: through their career, by donating, or by pursuing other projects or changes. It is about a way of thinking, rather than one particular way of doing good. You can read a good introduction to Effective Altruism here.