If an acid and a base are disolved in water the reaction can be written as:
H2SO4 + NH3 -> HSO4- + NH4+
On the other hand we can also consider:
H2SO4 + H2O -> HSO4- + H3O+
NH3 + H2O -> NH4+ + OH-
after which a neutralization between hydronium- and hydroxide-ions takes place
H3O+ + OH- -> 2 H2O
The net effect would be the same overall reaction.
I assume the second mechanism is more likely due to the great excess of water molecules, is this what actually happens?