The molecule that carries a positive charge on one side and a negative charge on the other side is said to be a polar molecule. This difference in charges is due to the difference in the electronegativities between the two atoms present in the molecule.
For example: Water (H2O) is a polar molecule.
Water consists of two atoms of Hydrogen and one atom of Oxygen. The electronegativity of Oxygen (3.44) is higher that of Hydrogen (2.2) due to which the oxygen atom attracts the electron towards itself thus carrying a negative charge more than the hydrogen atom. It is due to this difference in the electronegativities of the two atoms that makes water a polar molecule.