Acid strength is determined by the amount of that acid that actually ionizes. Acids are molecular covalent compounds which you don't expect to ionize (release an #H^+# and leave behind the conjugate base, or #Cl^-# for example).
The strongest acids ionize 100%. There are 6 that most consider to be the "STRONG" acids: HCl, HI, HBr, HNO_3#, H_2SO_4# and HClO_4#. These split apart 100%.
All other acids are weak acids and ionize to a much lesser amount. For example, acetic acid, like most weak acids, ionizes, 5%.