ethic vs ethics

I came across this article recently, and there is some good detail on the question here, which I think is articulated fairly well:

Ethic:

refers to the set of moral values and principles which, taken collectively, guide and influence the life of a group or, less commonly, of an individual. We may speak, e.g., of the Puritan ethic, i.e., the set of moral values and principles which guide the lives of those who, like the Puritans, attach value to austerity and the rejection of sensual and worldly pleasures; or of the work ethic, i.e., the set of moral values and principles which guide the lives of those who believe in the moral value of work.

Ethics:

(in the plural) is the code of behaviour considered appropriate to a particular group, especially a group defined by its occupation or profession. We may speak, e.g., of medical ethics, i.e., the code of behaviour that is considered appropriate for doctors, nurses, etc., in treating their patients; or of military ethics, i.e., the code of behaviour that is considered appropriate for soldiers and other members of the armed services. (You may like to think about the difference between military ethics and a military ethic.)