In an aqueous neutralisation reaction, acid and alkali solutions are mixed. The acid contains hydrogen ions, H+(aq), and the alkali contains hydroxide ions, OH−(aq). Neutralisation happens when these two ions react together to form water: H+(aq) + OH−(aq) → H2O(l).
This is the ionic equation for neutralisation. It leaves out the other ions, which are present in the solution before and after the reaction. These are called spectator ions. In hydrochloric acid reacting with sodium hydroxide, the sodium ions and chloride ions stay dissolved in the solution and together make the salt, sodium chloride.
The same ionic equation applies to any acid and any alkali in solution. This is why all neutralisation reactions of this type give water, and why the reaction removes the hydrogen ions that made the solution acidic and the hydroxide ions that made it alkaline.