Moral theories determine a theory of the right: they tell us what we ought to do morally. This is in contrast to theory of value, which focuses on what kind of things are morally good.
The theory of the right is typically rule-based: it may state how much value an action needs to produce to be permissible (“the right action is the one which maximises happiness”), or place restrictions or permissions on an agent which are separate to considerations of value (“you may never kill, even if doing so would maximise happiness”).
Effective altruism is focused on doing the most good, but this leaves open which constraints, if any, we should place on our pursuit of the good.
Different moral theories have different approaches to this question. Consequentialist theories say that placing constraints on our action is only justified if the constraints maximize the good in the long run. In contrast to this view, deontology, virtue ethics, and many other moral theories, argue that some actions are wrong, even if they would maximize good outcomes.
Alexander, Larry & Michael Moore. 2016. Deontological ethics. In Edward Zalta (ed.),
Stanford encyclopedia of philosophy.
Hursthouse, Rosalind. 2012. Virtue ethics. In Edward Zalta (ed.), Stanford encyclopedia of philosophy.
Sinnott-Armstrong, Walter. 2015. Consequentialism. In Edward Zalta (ed.), Stanford encyclopedia of philosophy.
Wikipedia. 2016. Normative ethics.