In 1 Samuel 11:8 Judah and Israel are mentioned. Please can you tell me what made up Judah? Was it the tribes of Judah and Benjamin. What then made up Israel? Was it the remainder of the tribes who inherited land both sides of the Jordan?
The reference in Samuel is not to the two kingdoms that began after the death of Solomon. The pasuk that you reference is due to another reason. Abarbanel says that the reference is because the tribe of Judah was always counted separately because of their strength and the fact that they would eventually produce the king of the Bnai Yisrael. Thus, the army of the tribe of Judah was counted apart from the rest of the nation.
After King Solomon died there was a rebellion against his son Rechavam. The "10 Northern tribes" seceded from the kingdom and formed their own under Yeravam ben N'vat. Benjamin and Judah remained loyal to Rechav'am. The Kohanim and Levi'im did not own land and remained connected to the Temple, though a number of Levi'im who lived in the Northern Kingdom were unable to move. A number of members of the ten tribes, remaining loyal to G0d, moved to the southern kingdom. The Northern Kingdom became known as "Samaria" after the capital city built by Omri.
Eventually, the northern kingdom was destroyed by Assyria and, in order to avoid rebellion, Assyria forced the residents of areas that it had conquered to be relocated elsewhere and the knowledge of where they are was lost.
The nonHebrews that replaced them became known as "Samaritans".
Since a number of members of the 10 tribes relocated to the kingdom of Judah, there were people who claimed to trace their ancestry to the original tribes, however in most cases that information is lost.