Different types of bonds form in chemical substances because of different characteristics of the elements. Ionic compounds form from a metal and a non-metal and result in the transfer of an electron from one atom to another. Covalent compounds result from the sharing of electrons between two atoms.
This difference in bonding results in different properties because of the way the electrons are arranged. Covalent compounds have lower melting and boiling points because they form molecules that are easier to separate from one another to take a substance into the liquid or gas phase. For ionic compounds, the bond strength between the ions is much greater and therefore those substances will have much higher boiling points and melting points.