Water is considered a polar molecule because it has a perceptible electromagnetic field that's grouped around a positive and a negative pole, like a magnet. This is a result of the special way that the atoms of a water molecule are grouped.
A molecule of water consists of two hydrogen atoms bonded to a single atom of oxygen. The bond between these atoms is covalent, which means that the oxygen atom donates one electron to each of the hydrogen atoms, and each of the hydrogen atoms donates its electron to the oxygen. This puts four of the molecule's eight electrons on one side of the molecule.
The other two electron pairs would normally drive the hydrogen atoms to opposite sides of the oxygen, which would equalize the molecule's surface charge and prevent a discernible asymmetry from forming. In the case of water, however, the unbonded electrons are interior to the bonded pairs. The energy balance of the electrons' negative charges thus drives the hydrogen atoms together on one side. The abundance of negatively charged electrons in this location gives the molecule a lopsided charge profile that forms a distinct polarity. The result of this is that water molecules are slightly attracted to each other like magnets, and that water is very good at dissolving other polar substances such as sugar and salt.