Covalent bond and ionic bond has a very basic difference that is presence of ions or not.
The ionic bonds are not limited to one ion only, actually they keep on joining again and again because they are held together by basically the electrostatic forces, due to the oppositely charged ions.
lets take example of NaCl molecule.
The positively charged ion will attract the negatively charged ions present around it thus a positively charged ion (see 1 in diagram) will have at least six negatively charged ions around it, and similarly one negatively charged (see 2 in diagram) ion will have six positively charged ions around it, so it makes a crystal as shown in picture. the ionic compounds do not exists as discrete molecule,but it exists as giant crystals.
On the other hand in case of covalent compounds the molecule do not have ions and the molecule remains confined to its atoms only and thus it do not make crystals.
Diamond and graphite is an exception and it is also due to the property of catination .
The Unconditional Guru 