Essential fatty acids are those that are required in the diet of a dog or cat because the animal is unable to synthesize the fatty acid.
Linoleic acid is essential for both dogs and cats. Cats also require arachidonic acid in their diet as well. Both linoleic acid and arachidonic acid are omega-6 fatty acids.
Read more at Pet Health Care Gazette: Dog and Cat Supplements: The Facts About Fatty Acid Supplements