A covalent bond is formed, when electrons are shared between atoms. When the atoms share two electrons, the bond is called single covalent bond. When the atoms share four electrons, the bond formed is double covalent bond. When atoms are joined together by covalent bond, they form molecules.
Ionic bond is formed, when one or more electrons are transferred from one atom to another. An atom that gains electrons has a negative charge and the atom that loses electrons has a positive charge.
Thus, the positively and negatively charged atoms are known as ions.