In the Arabic language the word "Seerah" comes from saara yaseeru. Linguistically, it means to travel or to be on a journey. When we’re talking about someone’s seerah we’re talking about that person’s journey through life. You are talking about the person’s birth, the events surrounding it, his life and his death, and you are also studying the manners and characteristics of that person. In modern times we still call it Seerah, like a resume is called a Seerah or Seerah Dhaatihi in the Arabic language.
In Islamic sciences or the Sharee’ah, Seerah means the study of the life of the Prophet Muhammed (sall Allahu 'alayhi wa sallam), the Last and Final Prophet and the Messenger of Allah. It is the study of his life and all that is related to him. The information related to him would be like the events and aspects surrounding his biography. This would include knowledge of events that preceded his birth, his interactions and dealings with his companions, his family, the people around him, and also the events that occurred shortly after his death. No book of Seerah stops at the death of the Prophet (sall Allahu 'alayhi wa sallam), it goes a little further in time or a few events further. Also, any Seerah book usually talks about the Prophet's (sall Allahu 'alayhi wa sallam) birth, his parents, his lineage, his tribe, and people that lived before him and the major events that took place around his birth. Usually when a book on someone’s biography is written it is imperative that a little information about the people that the subject interacted and death with. The Seerah of the Prophet (sall Allahu 'alayhi wa sallam) is no exception, it includes knowing about his companions, the disbelievers, the hypocrites, and the People of the Book that he interacted with.
May Allah’s Peace and Blessings be upon the Prophet (Sal Allahu Alayhi Wa Sallam), his family, his companions, and those who follow his path until the Day of Judgment. Aameen.