How is a strong acid different from a weak acid?

The difference between a strong acid and a weake acid is their ability to dissociate and form H+ions

The concentration of an acid (or any other solute) is measured in moles/litre (moldm-3). Strong and weak acids are defined by the ease with which they lose (or donate) hydrogen ions (protons). A strong acid, when placed in water, will almost fully ionise/dissociate straight away, producing H + (aq) ions from water.