Strong acid
(in water)
A substance which dissociates quantitatively into H3O+ in water, e.g. hydrochloric acid .
HCl + H20 
H3O+ + Cl-
Weak acid
(in water)
A substance which partially dissociates in water to give ions H3O+ , e.g. the balanced reaction for ethanoic (acetic) acid:
CH3COOH
+ H20 CH3COO-
+ H3O+