The 15th of the month of Adar is called Shushan Purim. Shushan was the capital of the Persian kingdom during the time of King Ahasuerus, when the miracle of Purim occurred, the salvation of the Jewish people from the hands of Haman, through Mordechai and Esther.
The Megillah tells us that the Jews outside the capital fought for their lives on the 13th of Adar and rested and celebrated their victory on the 14th of Adar. Meanwhile, the Jews in the capital fought on the 13th and 14th of Adar and rested on the 15th of Adar.
Based on this, our sages established the celebration of Purim for cities in general on the 14th of Adar, and the celebration for cities that in the time of Joshua bin Nun (when the Jewish people entered the Land of Israel after the exodus from Egypt) were surrounded by a wall (like Jerusalem), for the 15th of Adar, Shushan Purim.
There are cities in the Land of Israel about which there are doubts as to whether they had walls at that time (such as Tzfat, Tiberias and others) and therefore the Megillah is read on both the 14th of Adar and the 15th (in the second case without reciting the blessing).
For the other cities that were not on the Shushan Purim holiday, no supplications are recited in prayer, and it is customary to increase the amount of food one regularly eats.