Short answers, pulled from the story.
An alkene is a hydrocarbon containing one or more carbon-carbon double bonds, making it unsaturated, whereas an alkane has only single bonds. Alkenes are more reactive than alkanes because their pi bond is susceptible to addition reactions, and they generally have stronger odors than their alkane counterparts.
Ethylene, also called ethene in IUPAC nomenclature, is the simplest alkene. It is the organic compound produced on the largest industrial scale of any, serving as a monomer for polyethylene and as a signaling molecule in plant ripening.
In cis isomers, functional groups attached to the doubly bonded carbons sit on the same side of the double bond; in trans isomers they sit on opposite sides. These two forms interconvert so slowly at room temperature that they can be handled without spontaneous isomerization, because rotation around a double bond is energetically costly.
Alkenes are produced primarily by hydrocarbon cracking, in which ethane and propane in the United States and Middle East, or naphtha in Europe and Asia, are broken apart at high temperatures, often over a zeolite catalyst. Catalytic dehydrogenation, which strips hydrogen from alkanes at high temperatures, is a related industrial route.
Alkenes most commonly undergo addition reactions at the pi bond, including hydrogenation to give alkanes, halogenation to give dihaloalkanes, hydration to give alcohols, and epoxidation with percarboxylic acids. They also participate in ozonolysis to cleave the double bond, Diels-Alder cycloadditions, and polymerization to produce plastics.
Alkenes are used to produce polyethylene and polypropylene plastics via polymerization of ethylene and propylene. Vinyl chloride derived from alkene chemistry is a precursor to PVC, styrene feeds polystyrene production, and 1,3-butadiene is used to manufacture synthetic rubber.