When we mix acid and base why the result became salt and water?

A neutralization reaction is when an acid and a base react to form water and salt.

It involves the combination of H+ ions and OH- ions to generate water. 

When a solution is neutralized, it means that salts are formed from equal weights of acid and base.

  Acid + Base --> Salt + Water

Therefore salt is a combination of a metal ion and a non-matal ion. eg: NaCl, KCl.