A molecule is polar if it contains atoms of differing electronegativities arranged asymmetrically around a central atom. This causes an overall partial negative charge on one end of the molecule and an overall partial positive charge on the other end.
Molecules containing more than one type of atom consist of polar bonds. This is because different atoms have different electronegativities. Electronegativity of an atom is a measure of its tendency to attract a bonded pair of electrons in a covalent bond to itself. For example, when oxygen and hydrogen form a bond in a water molecule, the bonded electrons are more attracted to the oxygen atom because it has a higher electronegativity value, which makes the bond polar. However, in a chlorine molecule in which two chlorines are bonded together with a non-polar bond, the bonded pair of electrons is equally shared between the atoms because both chlorine atoms have the same electronegativity.
In addition to having polar bonds, polar molecules must also have the bonds arranged asymmetrically around the central atom so that the polarities do not cancel each other out. For example, in carbon dioxide, the individual bonds between the carbon and oxygen atoms are polar, but they are arranged symmetrically on either side of the central carbon, which cancels the net dipole moments of the bonds. In water, the polar bonds between the hydrogen atoms and the central oxygen atom are arranged in an asymmetrical bent conformation. This causes the oxygen atom to have a partial negative charge and hydrogen to have a partial positive charge, because oxygen attracted the bonded pair of electrons more strongly towards itself, which causes water to be polar.