I've heard omega-3s in fish are good for the heart. Why is this true?

Fish is a good source of protein and most varieties are free of or very low in saturated fats, which can contribute to high blood cholesterol levels.  Many types of seafood contain “good” kinds of fats, which are unsaturated fats, like omega-3 fatty acids.

Doctors at Harvard Medical School have found evidence to suggest that the omega-3 fats in certain types of fish can stop dangerous irregular heart rhythms, which can trigger heart attacks or sudden death. This study is one of the latest to highlight the potential health benefits of eating fish. Numerous other studies have shown that omega-3 fatty acids can protect against heart disease and stroke.  For these reasons, the American Heart Association recommends eating fish at least two times per week. http://www.americanheart.org/presenter.jhtml?identifier=3013797