Lipids

Lipids include fatty acids (breakdown products of fat) and cholesterol.

Fatty acids

can be divided into three main groups: saturated [1], mono-unsaturated and polyunsaturated. With these three the key is the ratio -- lots of saturated fatty acids, relative to the others leads to atherosclerosis (hardening of the arteries).

Their specific chemical structures are available in organic chemistry or biochemistry textbooks, in general they consists of a vary chain of carbon-atoms, with a carboxylic acid group at one end.

Fatty acids are used to make certain types of hormones, as well as to store energy. Two fatty acids, linoleic and alpha-linoleic acid, are essential for humans.

Omega-3, and -6

Some fatty acids are referred as "omega-3" and "omega-6". This refers to the location of a double bond from the final (omega) carbon atom in the fatty acid.

It is believed that omega-3 fatty acids, found in fish oil, may have beneficial health effects, but there is little hard evidence for this.

Cholesterol

can be made by the liver, so there is little dietary need. When we eat cholesterol, the liver compensates by making less. Contrary to popular belief, dietary cholesterol seems to have little effect on blood-cholesterol levels, instead saturated fatty acids seem to be more important.

Phytosterols

Not sure about these. Many be one and the same as phyto-oestrogens, plant mimics of the female sex hormone. Intake is believed to control symptoms of menopause.

Notes

[1]

`saturation' in this sense refers to hydrogen. Ask an organic chemist.