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 family is a mother, a father, and children. Many families do not have both parents as part of the family unit. These are known as single-parent families. In some cultures, a family includes grandparents, uncles, aunties, cousins and other people as part of the core family group.