For a strong acid, the equilibrium lies (strongly!) to the right. #HX# # (X =Cl, Br, I), H_2SO_4, HClO_4# are all examples of such "strong" acids. You should practise writing the equilibrium with these strong acids.
Weak acids are those acids for which the equilibrium lies to the left, the reactant side. These include #HF, H_3CO_2H, H_3PO_4, H_2S#. Do not be intimidated by #H_3O^+#, it is simply a water molecule with another #H^+# added (are mass and charge conserved; its formal name is the hydronium ion). In water, we conceive the acid species to be a cluster of water molecules (2 or 3 or 4 or 5) with an extra #H^+# associated.
For the reaction above, the representation,
#HA rightleftharpoons H^+ + A^-#, is entirely equivalent and acceptable.
To measure #H^+# or #H_3O^+# we use the parameter #pH#, short for pouvoir hydrogene, which is #-log_(10)[H^+]#.
