Essential fatty acids

Essential fatty acids commonly known as omega-3 and omega-6.

they are polyunsaturated fatty acids, 
omega translates as tail,
the number indicates where the first carbon double bond is (often referred to as nomenclature) 

As all fatty acids, there is a methyl terminus (CH3) and a carboxyl terminus (COOH)
e.g. for omega - 3, the double bond will be in the third position from the tail/methyl terminus of the fatty acid, giving us the name omega-3....

Metabolism of fat

they are known as essential fatty acids - as they cannot be metabolised by the human body. 
Synthesis can be via different starting metabolites.

Omega-6
these are found to be eaten more in the Western diet/world
they can be metabolised by the following pathway:
 LINOlEIC ACID (18: 2n-6) - 
GLA-
DGLA - 
ARACHIDONIC ACID (20:4n-6)

Omega-3
the high consumption of omega-6 to omega-3 is clear in the western world, mainly due to less consumption of fish.
It is metabolised in the following pathway 
a-linoelic acid (ala) (18:3n-3) - 
20:4n-3 - 
EPA (22:4n-3) -
DPA (22:5n-3) - 
DHA (22:6n-3)

Both of these processes go through a series of desaturation and elongation. 
They compete for the same enzymes through out the process, meaning that if dietary consumption of ALA or LA is higher than the other, one metabolism will be higher producing higher amounts of arachidonic acid or DHA.
In the western world, this is the case, as there is a higher consumption of LA, meaning there is an increased formation of arachidonic acid compared to DHA.

Deficiency of EFA's:
scaling dermatitis
rashes
poor wound healing
reproductive failure
renal failure
growth retardation

To avoid deficiency, EFA intake should be 1-2% of the daily energy intake