Narrowly, parents and their offspring. More broadly, the group defined as those who have common ancestry. That is, the group produced as a result of sex leading to offspring.
The term is also used more loosely to describe a group with some form of common characteristic or a group with a relationship (other than ancestral).
The traditional western view of a nuclear family is a mother, a father, and children. The traditional western view of the extended family includeds grandparents, aunts, uncles nephew, nieces and grandchildren. As single-parent family is one in which only one parent raises the children. In some cultures, a family includes grandparents, uncles, aunties, cousins and other people as part of the core family group.