Alkanes are saturated, alkenes aren’t
Alkanes undergo substitution reactions, alkenes electrophilic addition
Both of them have a homologous series, or a general formula, for alkanes, this is CnH2n+2, where n is the number of carbons in the chain, for instance, an alkane with 4 Carbon chain length will typically be butane, as where n = 4, it is C4H10 (2x4 + 2 =10)
For alkenes, this formula is just CnH2n, so where a 4 length alkane would be Butane, a 4 length alkene would be butene, as this is C4H8.
To test for alkenes, you place the hydrocarbon into bromine water, should the bromine water turn colourless, a dibromo reaction has occured, i.e, the double bond has essentially been split open and reacted, turning it colourless means an alkene is present (unsaturated)
If it remains orange, it will be an alkane or something else.