Omega-3s are a class of fatty acids found in fish oils (particularly salmon and other cold-water fish). These acids lower the levels of cholesterol and LDL (low-density lipoproteins) in the blood. (LDL cholesterol is the "bad" cholesterol.) EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid) are the two principal omega-3 fatty acids. The body has a limited ability to manufacture EPA and DHA by converting the essential fatty acid, alpha-linolenic acid (ALA) which is found in flaxseed oil, canola oil, or walnuts.
Omega-3 fish oil is considered a neutraceutical, a food that provides health benefits. Eating fish has been reported, for example, to protect against late age-related macular degeneration, a common eye disease. The maximum benefit appears to be from eating fish once a week.