An ionic bond is a strong electrostatic force of attraction between oppositely charged ion centres. Ions are derived from atoms that have lost or gained one or more electrons to become a cation or anion respectively.
Ionic compounds typically have a high melting point due to the high level of energy that is required to overcome the strong forces of attraction between ions.The higher the relative charges of the ions, the more energy is needed to overcome the attraction, so the melting point is higher.
They conduct electricity when in solution or molten as the ions are free to move, however, they do not usually conduct when in solid form.
Ionic compounds (such as NaCl) also tend to be soluble in water. This is due to water molecules being polar. The bonds between the ions first break (this is endothermic and is known as the lattice enthalpy of dissociation) and then bonds between the ions and water molecules are made (this enthalpy change is the enthalpy change of hydration).
Ionic compounds will usually have a degree of covalent character (electron sharing). Bonds are described as ionic when the ionic character is greater than the covalent character.
Solid ionic structures form a lattice. NaCl, for example, forms a giant ionic lattice. This consists of a basic unit of sodium and chloride ions repeated multiple times. In NaCl, the lattice is regular crystal structure as the ions are packed together. Other ionic compounds can have different types and shapes of the lattice; the structure of which decides their physical properties.