Ionic bonding is bonding between a metal and a non-metal. This happens due to a transferal of electrons. For example a metal from group 2 such as Magnesium bonding to a non-metal from group 6 such as oxygen. Magnesium will give its 2 outer electrons to oxygen. In doing this Magnesium now has a full outer shell with a charge of +2 since it lost 2 electrons and oxygen has a full outer shell with the charge -2 since it gained 2 electrons.It has a giant ionic lattice with strong electrostatic forces of attraction between oppositely charged ions. This means it requires a lot of energy to break the bonds. The buzzword phrase would be - Electrostatic forces of attraction between oppositely charged ions.