What is the molecular shape of a water molecule and why?

Around the Oxygen atom in the water molecule are pieces: 2 hydrogen atoms and 2 pairs of extra valence electrons. Each piece repels each other piece, so they try to get as far away from each other as possible. The resulting geometry is called a tetrahedron (picture a 4-sided pyramid with a triangular base) with the oxygen at the center. When we draw the water molecule, usually we don't draw the 2 pairs of valence electrons, so the molecule looks like a V.

note: because of the differing repulsive forces between a pair of valence electrons and a hydrogen atom, the actual geometry is not a perfect tetrahedron (central angle 109.47°), but very close (H2O central angle is 104.45°).