Fish oil is a common term used to refer to two kinds of omega-3 fatty acids: eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA). These omega-3 fats are usually found in fish, animal products, and phytoplankton.
The fatty acids EPA and DHA are involved in regulating various biological processes such as the inflammatory response, various metabolic signaling pathways, and brain function. They can be synthesized in the body from alpha-linolenic acid (ALA), but in small amounts for most people.
Fish oil causes a potent reduction in triglyceride levels, and a more modest reduction in blood pressure in hypertensives. Despite this, long-term trials haven't found a reduction in the rate of cardiovascular events.
It appears to notably improve mood in people with major depression, though it's unclear if it has an effect in people with minor depression. EPA, in particular, seems to be the most effective omega-3 fatty acid for this purpose which suggests that the effects of fish oil are due to reducing neuroinflammation. Its anti-inflammatory benefits also seem to extend to reducing the symptoms of systemic lupus erythematosus. However, its benefits shouldn't be assumed to extend to inflammatory diseases in general.
Many fish oil supplements may contain harmful lipid peroxides (oxidized lipids that can damage cells), but it's unclear if this has notable consequences to health.
"Fish oil" refers to a solution of fatty acids where the omega-3 fatty acids eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are dominant. Fish is by far the most common source for fish oil, hence the name, but an artificially manufactured EPA/DHA dominant mixture from any source could be considered fish oil. Typical fish oil can contain small amounts of other omega-3 fatty acids, usually DPA and fatty acids that don't belong to the omega-3 category. Alpha-linolenic acid (found most abundantly in nuts and seeds), is an omega-3 fatty acid that can be turned into EPA and DHA but is not itself a fish oil fatty acid.