Koseki is a family registry system used in Japan to record vital information about individuals and their families. It includes details such as births, marriages, deaths, and changes in family structure. Each household has a koseki, which serves as an official document for legal and administrative purposes. The koseki system helps the government maintain accurate population records and is essential for various processes, including obtaining identification documents and accessing social services. It reflects the traditional importance of family lineage and relationships in Japanese society.