An ionic bond is comprised of the electrostatic attraction of positively and negatively charges ions which holds them together. A common example of a compound held together by ionic bonds is table salt (NaCl), which consists of Na+ cations and Cl- anions held together in a solid crystal. The attractive force between positive and negative ions stabilizes the crystal.
It is important to remember that ionic bonds, unlike covalent bonds, are adirectional, meaning that ionic bonds occur between the ion and all other ions surrounding it. Hence ionic compounds do not occur as discrete units but as large aggregates. Furthermore, when ionic compounds are placed in water or other polar solvents they dissociate into their component ions. When you encounter ionic compounds in the context of organic reactions, they will almost always occur as free ions in solution. We will see that in the context of organic chemistry, covalent bonding is far more important than ionic bonding.