The R-group of a fatty acid may be saturated or unsaturated.

Fatty acids are made up of oxygen, carbohydrate and hydrogen.

In some all of the bonds are being used to join together the separate elements- these are called saturated.

In others not all of the possible bonds that can be made have been, these are unsaturated; this happens because carbons are double bonded (these can be made into single bonds freeing up another space.)

If a fat is unsaturated, its double bonds will cause kinks in the chain- this means that the fats cannot line up and compact into a solid, so they are liquid at room temperature.