The degree of ionisation or dissociation (which is the fraction of the total amount of the electrolyte ionised) of different electrolytes is different.
Certain electrolytes in solution ionise almost completely even at moderate dilutions. They are called strong electrolytes.
All salts, mineral acids like HCI, H2SO4, HNO3 etc. and the bases such as NaOH, KOH etc. dissociate completely in water and are strong electrolytes.
Electrolytes which ionise almost completely in solutions are known as strong electrolytes while electrolytes which ionise only to a small extent in solutions are known as weak electrolytes.