The degree of ionisation or dissociation (which is the fraction of the total amount of the electrolyte ionised) of different electrolytes is different. There are many substances which dissociate only to a small extent in aqueous solution. They are known as weak 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.Read more:
Difference between strong electrolytes and weak electrolytes