Electrons are negatively charged. If the two atoms share the electrons equally, then the negative charge is evenly spread out over the entire molecule, balanced out by the positive charges of the protons in the atoms' nuclei. This makes the entire molecule neutral - a non-polar bond, making a non-polar molecule. However, if one of the atoms is particularly greedy for electrons (called electronegativity), then its side of the bond will have more electrons. Therefore, that side of the bond will have a slightly negative charge, and the other side will have a slightly positive charge.