Water is considered to be made of polar covalent bond because the oxygen (a very electronegative element) pulls the electrons from the hydrogen. Electronegativity is the tendency of an atom to pull electrons from an other atom. This make the oxygen more negative and the hydrogen more positive. On the periodic table, oxygen is one the highly electronegative elements because it has 6 electrons in it's outer shell and it wants 8 electrons to be stable.
The octet rule in chemistry states that elements want 8 valence electrons in their outer electron shell. In terms of electronegativity, the closer you are to having 8 electrons the more electronegative the atom becomes.
In the case of water, oxygen uses the 2 electrons from the 2 hydrogens (1 valence electron) to make 8 valence electrons in its outer shell. The pull of the two electrons from the hydrogens make the hydrogen more positive than negative and oxygen more negative than positive.