The major difference between covalent and ionic compounds is in their bonding:

Covalent Compounds:

Bond Formation: Covalent bonds are formed through the sharing of electrons between two atoms, generally between nonmetals.

Properties: These materials are generally of lower melting and boiling points. They can be found in the form of solids, liquids, and gases at room temperature, and they are bad conductors of electricity as they do not contain free ions.
Example: Water (H₂O), in which oxygen and hydrogen share electrons.

Ionic Compounds:

Bond Formation: Ionic bonds are created when one atom loses an electron to another, thus forming cations with a positive charge and anions with a negative charge. Usually, these are the metals and nonmetals.

Properties: Ionic compounds typically have very high melting and boiling points, are solid and crystalline at room temperature, and because of the existence of free-moving ions, they can conduct electricity when dissolved in water or the molten state.
Example: Sodium chloride (NaCl), which is a case of sodium transferring an electron to chlorine.

The reason for the difference is that in the case of covalent compounds, the electrons are shared whereas in ionic compounds the electrons are transferred.