There are two kinds of essential fatty acids: omega-3, and omega-6.
If you study the biology of nutrition, you'll find that the names of fatty acids are very similar to one another. Keep this in mind.
The lowercase omega in Greek is our w. The most basic form of fat is the triglyceride, which contains a glycerol (yes, alcohol) attached to three fatty acids, similar to the letter E.
Omega refers to the numbered bond from the designed end of the fatty acid that is the first carbon-carbon double bond. Imagine a straight line with dots in the middle representing carbon with a COOH (carboxyl) on the end. That's a fatty acid. Now count back three bonds from that COOH, and all omega-3s have their first double bonded carbon at that bond. Omega-6 has the first one at the sixth bond, and omega-9s at the 9th.
There are two fats that humans cannot synthesize from any other fatty acids. They are alpha-linoleic acid (ALA), the smallest Omega-3 fatty acid, precursor to all other omega-3 fatty acids, and linoleic acid (LA), the smallest omega-6 and precursor to all other Omega-6 fatty acids. The human body lacks the desaturase enzymes to break it apart other fatty acids for reconstituion into Omega-3 and Omega-6 fatty acids.
Remember that Omega-6 and Omega-3 fats are types of polyunsaturated fats, that is, they have more than one double bonded carbon in their fatty acid structure.
Omega-9 fatty acids, with the exception of one (Mead acid), are monounsaturated fatty acids. The most famous Omega-9 is oleic acid, the main component of olive oil. Sound familiar?
Polyunsaturated fats generally degrade at low temperatures. Hence why you do not cook with them. Monounsaturated fats generally fare slightly better, but cooking with them still depends on the composition of the oil. Saturated fats are generally the most stable for cooking, as are trans fats. But trans fats are bad for you
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Oleic acid, the principle omega-9 fatty acid in olive oil, is the monounsaturated form of steric acid, which comprises between 15-20% of beef fat. Yes, beef fat is only around 50% saturated fat, the other 45% is monounsaturated (at least in grass-fed beef), and the final 5% is polyunsaturated. The other primary fat is palmitic acid, the most common saturated fat.
The body primarily uses three omega-3 fatty acids: ALA, EPA (eicosahexaeonic acid), and DHA (docosahexaeonic acid), which increase in size order respectively. Flax seeds contain more omega-3s than any fish, and all in ALA form. The other two are used only by animals, and are the two princple forms found in fish oil. Omega-3s, curiously, are found in the thylakoid membranes of the leaves of broadleaf plants. That's why purslane is so good for you. Omega-3 fatty acids are required for proper formation of cell membranes, neural connections, and lots of other stuff. It is controversial that babies cannot produce EPA or DHA until older, and so vegan babies need supplementation. I disagree with this - breast feeding is the way to go until they can make it.
The vast majority of the western world consumes too much omega-6 and not enough omega-3s.