 First of all dissociation. When an acid is dissolved in water it will dissociate and release hydrogen ions. If the molecule which is originally neutral has lost a positively charged hydrogen ion then what is left must be an anion. For instance hydrochloric acid will dissociate to give a hydrogen ion which is positive and a chloride ion which is negative. Acetic acid which is the acid in vinegar has this structure. When it dissociates it gives a proton and the acetate ion. Sulfuric acid dissociates to give two hydrogen ions and a sulfate ion. In fact this dissociation can happen in two steps. First the sulfuric acid dissociates to give one proton and the hydrogen sulfate ion and then the hydrogen sulfate ion can itself act as an acid dissociating to give a second proton and the sulfate ion. So is the acid donating its proton? Well it's certainly losing it and if you remember the proton does not hang around by itself for very long. It bonds with a water molecule to form a hydronium ion. So the acid is donating its proton to a water molecule and the water molecule is accepting a proton so in this context the water is acting like a base.