The active participle اسم الفاعل
In English one who reads is a "reader" and one who writes ia a "writer". In Arabic a noun on the pattern of "FA3IL"فاعِل is derived from the verb to denote the one who does the action
كتب he wrote كاتِب writer
سرق he stole سارِق thief
عبد he worshipped عابد صخقساهححثق
To know more about this subject click on the Title اسم الفاعل where Mr Foda explaining in Arabic.