What are covalent bonds?

Covalent bonds are a type of chemical bond formed by a shared pair of electrons between atoms. 

Covalent bonds tend to occur between non-metal type atoms (metals tend to form ionic bonds which are formed by a transfer of electrons from one atom to the other rather than a sharing of them). 

There are single covalent bonds- each atom contributes one electron to the shared pair.

There are also multiple covalent bonds- each atom contributes more than one electron to form 2 or sometimes 3 pairs of electrons.