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.
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.