Essential fatty acids are made up of two families Omega 3 and Omega 6 and are all considered polyunsaturated fatty acids. Omega 3 fatty acids are made from Alpha Linolenic Acid (ALA) and Omega 6 fatty acids are made from Linoleic Acid (LA).  Consuming both these allows the body to make the other EFA's it requires.   From Alpha Linolenic Acid the body can produce Docosahexaenoic Acid (DHA) and Eicosapentaenoic Acid (EPA); from Linoleic Acid it is able to produce Arachidonic Acid (AA) and Gamma Linoleic Acid (GLA).  Omega 3 and Omega 6 are not interchangeable; we must consume both to maintain health.