Hydrochloric acid, or HCl, is a strong acid and acetic acid, or CH3COOH, is a weak acid. This is because HCl can be said to dissociate (split down into smaller molecules H+ and Cl-) completely in water, while CH3COOH dissociates only partially/a little. That means HCl would break down into more H+ ions than CH3COOH of the same concentration. Since pH is determined by the H+ concentration (higher H+ concentration = lower pH), HCl is a stronger acid.