"Fish oil is oil derived from the tissues of oily fish. Fish oils contain the omega-3 fatty acids eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), precursors of eicosanoids that are known to reduce inflammation throughout the body, and are thought to have many health benefits.
Fish do not actually produce omega-3 fatty acids, but instead accumulate them by consuming either microalgae or prey fish that have accumulated omega-3 fatty acids from microalgae. Fatty predatory fish like sharks, sword fish, tilefish, and albacore tuna may be high in omega-3 fatty acids, but due to their position at the top of the food chain, these species can also accumulate toxic substances (see biomagnification). For this reason, the U.S. Food and Drug Administration recommends limiting consumption of certain (predatory) fish species (e.g. albacore tuna, shark, king mackerel, tilefish and swordfish) due to high levels of toxic contaminants such as mercury, dioxin, PCBs and chlordane. Fish oil is used as a component in aquaculture feed. More than 50 percent of the world's fish oil used inaquaculture feed is fed to farmed salmon.
The omega-3 fatty acids in fish oil are thought to be beneficial in treating hypertriglyceridemia, and possibly beneficial in preventing heart disease. Fish oil and omega-3 fatty acids have been studied in a wide variety of other conditions, such as clinical depression, anxiety, cancer, and macular degeneration, although benefit in these conditions remains to be proven."
-taken from Wikipedia