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Out of all saturated and unsaturated fatty acids, your body cannot make only two kinds: so called Omega-6 (or EFA6) and Omega-3 (EFA3) unsaturated fatty acids. And if they are deficient in the diet, it can cause many symptoms and serious health conditions.

Omega-6 are polyunsaturated fatty acids - where "polyunsaturated" indicates that they have more than one double bond, and '"6" indicating that their first double bond is located at the 6th carbon atom from fatty acid's methyl group end (CH3, carbon atom single-bonded to three hydrogen atoms, also called omega, or ω group). Short-term Omega-6 deficiency can cause skin eruptions, hair loss, liver and kidney degeneration, and suppressed immune-function, among other symptoms; its prolonged absence is fatal.

On the other hand, excessive intake of Omega-6 fatty acids, even those not degraded by processing, can produce oxidative/inflammatory damage by stimulating production of inflammatory leucotrienes, and may promote clot formation, especially if the Omega-3 are deficient.

Omega-3 are also polyunsaturated fatty acids, but with their first double bond at the 3rd carbon atom from their omega group end. Their deficiency could cause growth retardation as well as neural dysfunction on various levels; also, they have anti-inflammatory and anti-clotting properties, and it seem to be playing important part in protection from cardiovascular diseases.

In the absence of dietary EFA intake, the body can partly compensate for their deficiency by inserting double bonds in other fatty acids, primarily those that are monounsaturated, like oleic acid, the most abundant fatty acid in olive oil. This adds to the health benefits of this type of dietary fat, but neglecting dietary intake of EFAs on that basis is not advisable.

There are several main molecular forms of both, EFA6 and EFA3, based on the size of their molecules (i.e. on the length of their carbon chain). According to it, they are classified as short-, medium- and long-chain EFAs, with the longer forms being also more unsaturated.

The shortest EFA6 forms, containing 18 carbon atoms, are Linoleic acid (LA, 18:2ω6, where 2ω6 means that it has two double bonds, with the first located at the 6th carbon atom from the omega group end), and Gamma-linolenic acid (GLA, 18:3ω6). The two main longer-chain molecular forms are Dihomogammalinolenic acid (DGLA, 20:3ω6) and Arachidonic acid (AA, 20:4ω6).

On the other hand, the shortest EFA3 form, also with 18 carbon atoms, is Alpha-linolenic acid (ALA or LNA 18:3ω3), with the main longer forms being medium-chain Eicosapentaenoic acid (EPA, 20:5ω3) and long-chain Docosahexaenoic acid (DHA, 22:6ω3). The abbreviations are commonly in use - you can see why.

These different EFA molecules have their specific functions in the body, primarily at the cellular level. It includes vital role in energy production, transfer of oxygen, cellular membrane function and cell division.

In other words, we need all EFAs. By adding carbon groups and/or inserting double bonds via desaturase enzymes, healthy body can produce longer-chain EFAs from their basic short-chain forms, Linoleic and Alpha-linolenic acid, which are the two basic EFA forms found in plant food sources. The two main EFA forms found in animal food sources (mainly fish and marine animals) are the medium-chain EPA and long-chain DHA. Some meats also have relatively significant Linoleic acid content.

Essential fatty acids DRI (Dietary Reference Intakes, the most recent set of dietary recommendations set by the government) for an average healthy adult is set at 5%-10% of the total caloric intake for Omega-6, and 0.6%-1.2% for Omega-3.

According to Udo Erasmus, probable optimum for the Omega-6 is
3%-6% of the total caloric intake, and 2% for the Omega-35. Within the "acceptable" total fat intake, which is, according to the government, in the 20-35% of total calories range (unbiased educated opinion is that it shouldn't exceed ~25%), that places the essential fatty acids portion at nearly 1/4 of the total fat intake.
 

If your enzymatic conversion from short to longer-chain EFAs is inefficient - either due to advanced age, genetics, nutritional deficiencies (specifically, vitamin B6, magnesium and zinc, needed for the key conversion enzyme, delta-6-desturaze), extensive oxidative damage at the cellular level, or disease - you need to secure direct intake of the longer-chain EFAs through diet or (likely) supplementation, or your health will suffer.