Ethics is the study of morality in an objective or socially universal context. It looks at questions such as whether or not a given moral system is harmful or beneficial, whether or not given moral value are helpful, and so forth.
The words "ethical" and "unethical", however, are almost always used in a rather different sense in which they indicate adherence to or violation of a professional code of ethics, i.e. the moral system of a given professional community. Professional ethicality is ultimately based on harm or benefit to the profession's credibility, which in turn is usually based on some function of the overall benefit or harm to society of a given rule or action.