What is an acid-base titration?

A titration is way to find an unknown concentration of a sample through a chemical reaction. 
The most common titration involves an acid and base to react. In order to set our experiment a standard is needed, and that can be either the base or the acid of our reaction. what does the word standard mean? To make it simple, a standard is something that can be reliable about its concentration. Sometimes a secondary standard can be used once it has been titred against a primary one. For istance , its very useful having a HCl as a standar but it doesn't below to the primary standards' category, so it can be titred against Na₂CO₃ and used then to find out about an unknown concetration of a NaOH solution.

The reaction involved is : NaOH+ HCl = NaCl + H₂O -> final pH = 7

if we have 20 ml of HCl 0.5 M in a beaker and the final volume of NaOH is 30 ml and the end of our titration, the NaOH concetration can be found as follow :

C_HCl  * V _HCl = C_NaOH* V _NaOH , so 0.50.02 = 0.03 * X->X= (0.50.02)/ 0.03 = 0.03 = NaOH concentration.

How could we see the end of this reaction? 
answer : looking at the final pH and using few drops of an indicator . An indicator is simply a molecule that changes colour once the reaction is completed and the pH of 7 is reached (in our case)? An example for our case? phenolphthalein!