Ethics was the study of morality. Every society had a different set of social values and mores that defined what was ethical and what was not; in some cases, two cultures' ethics could be so radically different that conflict was inevitable. This article covers the variations in ethics between societies.
The Ferengi valued profit, and the ethical standards (or at times lack thereof) in their society reflected this. For example, when Quark was selling the vacuum-dessicated remains of the entrepreneur Plegg, Odo found Plegg very much alive; rather than pressing charges or expressing outrage, Plegg found this amusing. (DS9: "Body Parts") One of the Rules of Acquisition stated "Treat people in your debt like family: exploit them." (DS9: "Past Tense, Part I", "The Darkness and the Light")
In 2369, while Keiko O'Brien was on leave on Earth, Miles O'Brien took charge of her school on Deep Space 9. One of the homework projects he assigned to the older students was to write an essay defining what ethics was. Nog claimed Vulcans had stolen his homework as they did not have ethics. (DS9: "The Nagus")
Denobulan medical ethics had the will of the patient rather than the necessity of providing a cure if possible at its core. Dr. Phlox summed this up with the statement "Hippocrates wasn't Denobulan." (ENT: "The Breach")