What is the basic difference between a single bond and a double bond?
A "single bond" is the primary bond between two elements. A "double bond" is formed between some elements when their electron structure allows for a second set of electrons to bond (the 'pi bond').
The available electrons for bonding between elements depends on their electronic structure. This is formalized in the Periodic Table for easy reference. All elemental bonds must contain at least one 'sigma' or single bond. Availability (electronically and spatially) of other electrons allows some elements to form multiple bonds.
For example, carbon and nitrogen can form, one, two or three bonds to one other element as well as up to three or four single bonds to three or four different elements. The number of bonds does NOT predict whether a bond is covalent or not. The electronegativity difference of the atoms must be compared for that determination.