A saturated compound is a compound in which the carbon atoms are bonded to one another by single covalent bonds only, never by the more reactive double or triple bonds. For this reason, they tend to be unreactive. A simple example is ethane (C2H6). In a molecule of ethane, each carbon atom is bonded to three hydrogen atoms and to the other carbon atom by single bonds.
An unsaturated compound, by contrast, is a compound in the molecule of which two or more carbon atoms are linked by double or triple bonds. Simple examples are ethene (ethylene) and ethyne (acetylene). By breakage of their double or triple bonds, unsaturated compounds can add on more atoms to their molecules – and for this reason are chemically reactive.