Carbon has four valence electrons in the outer shell but it forms three covalent bonds with another carbon atom. i.e Single bond, double bond and triple bond.
The single bond is called Alkanes e.g. Methane, Propane etc.
Double bonds are called alkenes e.g. Ethylene, Propylene etc.
Triple bonds are called alkynes e.g. Ethyne, Propyne, Butyne etc.