Ionic solids which dissolve in water are not the only substances which dissociate into ions.
Such solids or salts may be considered to be the products of reactions between two other classes of compounds which also exhibit dissociation behavior in water. These compounds are known as acids and bases. Like the salts which they form when they react, some acids and bases dissociate 100% and others, like the insoluble salts, dissociate to a very small extent. Thus some may be treated simply (the STRONG acids and bases) and others require treatment as equilibrium systems (the weak acids and bases).
So just what are acids and bases? These compounds are common substances which we encounter in everyday existence. They have been known for many years and the original descriptive definitions which have come down to us are still useful and interesting to look at.