In an ionic bond, atoms transfer electrons to become ions, which are charged particles. For example, sodium is a neutral atom and so is chlorine.However, sodium transfers the electron in its outer valence shell to chlorine. In doing so, sodium becomes positvely charged and chlorine becomes negative. In covalent bonds the bonds are not transferred, but rather, they are "shared" between atoms. In water molecules, hydrogen and oxygen atoms do not transfer their electrons. They share them. However, the oxygen is a little more electronegative than hydrogen so it "hogs" the electrons giving it a partial negative charge and making water a polar molecule but that's a whole different topic.