Carbon-carbon bonds

A carbon atom has 4 valence electrons

A single bond

Most bonds that carbon form are single bonds. Single bond simply means sharing one pair of bonding electrons between two atoms and is represented by one line. 

Two carbon atoms forming a single bond

In ethane, two carbons atoms share a single bond

A double bond

A carbon atom can also form multiple bonds with other carbon atoms.

In a double bond, two pairs of electrons are shared.

In ethylene, a double bond is formed between two carbon atoms

A triple bond

In a triple bond, three pairs of electrons are shared

In ethyne, a triple bond is formed between two carbon atoms