The long chain omega-3 poly-unsaturated fatty acids (omega-3 LC-PUFAs), such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), contained in fish oil are particularly important for good health. Typical Western diet may not be enough of the omega-3 LC-PUFA. People can remedy their omega-3 LC-PUFA deficiency by eating more fish or taking fish oil capsules.

DHA

DHA is an omega-3 LC-PUFA with 22 carbon atoms and 6 methylene-interrupted cis-double bonds. DHA is important for prenatal development and the nourishment of infants. This fatty acid seems to ensure healthy development of the brains, nerves and eyesight. 
 

EPA

EPA is an omega-3 LC-PUFA with 20 carbon atoms and 5 methylene-interrupted cis-double bonds.
The presence of EPA in the diet seems to promote antithrombotic processes in the body. Studies among the people having a traditional marine diet, rich in EPA, show a low evidence of coronary heart diseases.