It is all to do with the electrons which either transfer or are shared between atoms. When non-metals react together, they share electrons to form covalent bonds, but when a metal reacts with a non-metal, electrons transfer from one atom to another and this is called ionic bonding. In ionic bonding, metals always lose electrons forming positive ions whilst non-metals always gain them to form negative ions. The opposite charges attract and it is that attraction that is the ionic bond.