Hydrocarbons are compounds that consist of only the elements carbon and hydrogen in their molecular formula. They are largely compounds found in petroleum deposits and represent the decayed remains of once-living plants and animals. They are used widely in the energy industry in the production of various fuels such as gasoline, diesel fuel, and kerosene.
Hydrocarbons fit into two categories, saturated and unsaturated. The "saturated" designation means the carbon atoms in the compound have as many hydrogen atoms as they are allowed. Take the simplest hydrocarbon, methane, for example. There is one carbon and four hydrogens, which is the maximum bonding for each of the carbon atom's four single electrons. There is no room for anything else to attach to the carbon atom, so to get a reaction to take place, one must remove one of the hydrogens already present.
Unsaturated hydrocarbons, however, are hydrocarbons that have at least one or two double or triple bonds between the carbon atoms. The double bond indicates a vacancy where two electrons could potentially bind to the carbon atom. It is a simple matter to get the unsaturated hydrocarbon to "relax" the double bond in a chemical reaction, so that another element may bind to it. Therefore, unsaturated hydrocarbons tend to be more reactive in nature than do saturated hydrocarbons.