Compounds that have the same atoms (molecular formula) but different in the connectivity between the atoms are constitutional (formerly 'structural') isomers.
Hydrocarbon molecules containing no functional groups and the formula CnH2n+2 when fully saturated with hydrogen. examples, methane, ethane, butane, octane, isooctane
alkanes that contain a ring or loop of carbons. These can be monocyclic or contain several rings. The formula for saturated alkanes cannot be used as a ring can only be formed by removing a hydrogen from each end of a chain and connecting them covalently to make a ring. It is unsaturated therefore and is CnH2n like alkenes with only one pi bond and no rings.
aka olefin: a hydrocarbon molecule containing at least one C=C pi bond