There are many different styles of Arabic calligraphy. These developed over centuries into formal scripts. Each script has distinctive shapes and characteristics.
Scripts were created for various purposes. Some were used to write the Koran; others were used for court documents. Small scripts were created to send mail by pigeon post; large scripts were developed for architectural inscriptions; and so on.
Different regions developed their own styles. Three regions in particular were important in the development of calligraphy: the Arab world (from Morocco to Iraq), the Ottoman empire (present-day Turkey and beyond), and Persia (present-day Iran and beyond).