Ionic and molecular compound are the two main variety of compound .Every thing in this world is either classified as ionic or molecular compound.The difference between ionic and molecular compound lies in the nature of bonding between the elements of the compounds.

Ionic compound are the compounds having the Ionic bond between its elements.

First we need to understand Ions  to understand the Ionic bond.

 Ion:-

An ion is an atom or molecule where the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge.

An ionic bond is a type of chemical bond that involves a metal and a non-metal ion (or polyatomic ions ) through electrostatic attraction. In short, it is a bond formed by the attraction between two oppositely charged ions.

The metal donates one or more electrons, forming a positively charged ion( or cation) with a stable electron configuration(completing its octate). These electrons then enter the non metal, causing it to form a negatively charged ion (or anion) which also has a stable electron configuration(completing its octate).The electrostatic attraction between the oppositely charged ions causes them to come together and form a bond.

Molecular Compound are also know as Covalent compound because they have covalent bond between its element. 

Covalent bonds are the chemical bond in which pair of electrons are shared between the atoms and other covalent bond to fulfil its octate partially.

In the molecule H2, the hydrogen atoms share the two electrons via covalent bonding. Covalency is greatest between atoms of similar electro-negativities.

covalent bonding does not necessarily require the two atoms be of the same elements, only that they be of comparable electro-negativity.

Only covalent bond will be formed between atoms of same element.

1)Ionic compound has Ionic bond between its elements whereas molecular compound has covalent bond between its element.

2)Ionic compound are good conductor of electricity in liquid or aqueous state whereas molecular are not.

3) ionic compounds are formed between metals and non-metals whereas Molecular compounds are formed between two non-metals.

4)Ionic compounds are always solid and crystalline in appearance whereas molecular compounds can be in any physical state – solid, liquid, or gas.

5)Ionic compound are found in lesser number then molecular compound.

6)Ionic compound formed when overall energy change for the reaction is favourable – when the bonded atoms have a lower energy than the free ones whereas the strength of covalent bond(of molecular compound) depends on the angular relation between atoms in polyatomic molecules.