What do we call a molecule with oppositely charged ends, or poles?
Polar molecules have slightly positive and slightly negative ends. This arises from polar bonds, which come from an unequal distribution of electrons within a covalent bond.
Electrons might be unevenly distributed within the bond due to a difference in electronegativity. For example, fluorine
Now in a molecule with multiple covalent bonds, e.g. water, we must consider the direction of the dipoles to see if there is an overall polarity. If the dipoles face each other and cancel out, or face opposite directions and cancel out, there is no overall net dipole and thus the molecule is non-polar. If the dipoles do not cancel out, an overall net dipole exists and the molecule will be polar.
We see here that water has 2 polar bonds, two O-H covalent bonds in which the oxygen is much more electronegative than the hydrogen, resulting in the hydrogen being slightly positive and the oxygen being slightly negative . Now, from this diagram we also see that the bonds are positioned in a way such that the dipoles do not cancel out ( asymmetrical ). This results in an overall net dipole, meaning that water is a polar molecule.