To review, covalent bonds are the sharing of pairs of electrons between atoms, ionic bonds are when one atom like a metal loses its valence electrons to have a full outer shell donating the electron(s) to something that needs electrons to fill its outer shell then the two things are attracted by electrostatic forces (positive and negative attracting).

The metallic bond is similar to the ionic bonding in that metals (which never have a full outer shell in their elemental form) lose electrons so that they can have a full outer shell then they are positive, but the electrons they lose don't go to fill another atoms shell, the electrons form a 'sea' of electrons. So you have a bunch of positively charged metal cations separated by a sea of negative charge which is the electrons (remember a positive and positive repel each other so floating in the sea of electrons with full outer shells, metallic bonds are stable). Try to explore the next link and you will see nice pictures about metallic bonds.

A metallic bond is very much like a covalent bond, where several atoms share several electrons. These electrons are called 'delocalized' since they are not associated with one particular atom. An example of a covalent bond is a hydrogen molecule where two hydrogen atoms share two electrons. In a metallic bond, instead of having only a few atoms sharing some of their electrons, the whole material shares all the delocalized electrons. This means that these electrons can easily travel from one part to another within the material. That is why metals are good conductors of both, electricity and heat.