When an acid neutralizes a base?

Answer 1

It donates a proton (H+) to that base that's going to accept it:

(the proton from acetic acid is bold to recognise it and see is move to the N atom of ammonia)

CH3COOH + NH3 --> CH3COO- + H+NH3 (this last ion (ammonium) should be written as NH4+)

Answer 2

When an acid neutralizes a base, they react together to form water and a salt. The acid donates a proton (H+) to the base, resulting in the formation of water and a salt compound. The pH of the solution changes to a more neutral value as the acidity of the acid and the alkalinity of the base are balanced out.