Covalent bonds form the base of the chemical reactions between atoms of different elements. The only way these bonds can form is when an electron becomes shared by two elements, creating the connection that then results in a new substance. Covalent bonds can exist as polar or nonpolar compounds, but it's important to note that all bonds that are polar or nonpolar in nature must also be covalent.
You can identify a polar bond by looking for bonds that unequally share electrons. This happens when two different atoms come together from two different elements, as opposed to two atoms from the same element, which do not form polar bonds. The reason for polar bonds is that every atom pulls electrons at its own level, which means that unless the elements are the same, one atom of an element will be better at pulling electrons than the other in the bond.
A nonpolar bond is the name given to a bond where two atoms share electrons equally with one another. These bonds only occur when two atoms are from the same element, since only matching elements will have the exact same ability to pull electrons. An example of this would be H2 or O2, since the bond is still of only one element.
The primary difference between polar and nonpolar bonds remains the sharing of electrons. When equally shared between atoms, the bond is nonpolar, but if the electrons are not equally shared then it's polar. The easiest way for determining this is by looking at the atoms' bonding. If they are all one element, then the bond is nonpolar, but if the elements are different, like H2O, then the bond is polar.