An acid which dissociates almost completely in an aqueous solution, thereby producing high concentration of H+ (aq) ions is called a strong acid. For example, hydrochloric acid, nitric acid, sulphuric acid and phosphoric acid.
An acid which dissociates only partially in an aqueous solution thereby producing low concentration of H+(aq) ions is called a weak acid.
For example, acetic acid, carbonic acid, sulphurous acid, nitrous acid are weak acids.