Molecular dipoles exist if one or more of the atoms is more electronegative than the other(s)
The most common dipole is water.
Since #O# is more electronegative than #H# the shared electrons tend to be more in the neighbourhood of the #O#-atom.
Since the molecule is 'bent' they tend to be more in the upper part of the figure above. This gives a slight negative charge (called #delta-#) at the top, and a #delta+# at the #H#-arms.
Since #+# and #-# attract, the next molecule will tend to turn one of its #H# 's toward the #O# of the first one.
This polarity also has many consequences for the behaviour of a polar liquid like boiling point, solvability, etc. and even the shape of snow flakes.
Dipoles occur when there is a relative charge on each side of a molecule induced by the bond electronegativities.
The dipole moment of an entire molecule is made up of two bond moments -vector quantities having both magnitude and direction. Thus, a measured dipole moment is equal to the vector sum of the bond moments that comprise it.
Bond moments are vector quantities, possessing both magnitude and direction. Therefore, it's possible for a molecule to have bond moments and yet to be non-polar, if the individual bond moments in the molecule are equal in magnitude but opposite in their direction, therefore cancelling each other out. Thus, the sum is 0 and there is no dipole moment. An example of a molecule with bond moments and that is non-polar is CO2. You could have a linear molecule with the same atom on either side of the central atom that cancel each other out.