What is meant by the term ion?
An ion is used to describe a "charged particle." The particle/atom can either be positively or negatively charged. Cations (+ve ions) have lost electrons to become positive, e.g Al=> Al3+ . Similarly, anions (-ve ions) have gained electrons, e.g Cl=> Cl-
The A- ion is the generic term for the conjugate base of an acid. The charge is balanced by an H+ ion. The identity and formula of this ion depends on the acid, for example, if the acid is acetic CH3CO2H acid, A- is the acetate ion or CH3CO2- If it is sulfuric acid (H2SO4) A- is the bisulfate ion HSO4-