According to Quinn ("Ethics for the Information Age", 5th ed., p. 57)
Ethics is the rational, systematic study of conduct that can cause benefit or harm to other people. Because ethics is based on reason, people are required to explain why they hold the opinions they do. This gives us the opportunity to compare ethical evaluations. When two people reach different conclusions, we can weigh the facts and the reasoning process behind their conclusions to determine the stronger line of thinking. It's important to note that ethics is focused on the voluntary, moral choices people make because they have decided they ought to take one course of action rather than an alternative. Ethics is not concerned with involuntary choices or choices outside the moral realm.
This raises several important questions:
The apparent answer for Quinn is behavior that can cause benefit or harm to others. But this may be too open-ended. Does benefit and harm include effects on our world view? That is, do we count being inspired or offended as part of the benefits or harms? Or should we restrict our attention to clear-cut material benefits?
We have a lot of effects on other people that they may consider to be beneficial or harmful. Should we draw a line somewhere? Where?
Quinn does give examples to address this question. Both concern driving.
Can you fault the driver in the first case for not being aware of the second pedestrian? Generally, to what extent is an action involuntary when it depends on how well we have prepared ourselves to deal with situations?
Do we need ethics just to be sure everyone is behaving correctly? Or is our own ethical behavior an important part of our sense of self-worth?