In the formation of an covalent bond, the electrons are shared between the atoms involved in the bonds. This way, the electron lies with both atoms, and is not completely transferred, meaning the there is no overall effect on charge.
Contrarily, in ionic bond formation, an electron is completely donated from one atom to another. The atom donating the electron is left with a positive charge due to loss of an electron, while the atom receiving the electron is left with a negative charge due to the gain of an electron.
