What are hydrocarbons and why are they so important?

In an ever increasing 'green' world hydrocarbon use has become a big deal. Hydrocarbons extracted from oil, taken from the earth has been linked to global warming and in recent years an approach to minimise hydrocarbon use and methane production has been undertaken.
Firstly hydrocarbons are defined as molecules only containing carbon and hydrogen, hence -hydro and -carbons. They are important for fuels, fragrances, detergents and many other things.
An important member of the hydrocarbons are alkanes and alkenes. Alkanes (saturated) consist of either a branched or cyclic hydrocarbon chain. Alkenes (un-saturated) possess at least one double bond between two carbon atoms within the molecule. This results in a totally different way in which the molecule reacts.
This difference in reactiveness is the reason why saturated fats in the body are bad for us and un-saturated are good.