If it is a father's first cousin, then that cousin is a first cousin once removed to the father's children. This means that there is a one generation difference between two family members. Similarly, the term "once removed" can refer to other family members of different generations.
A grandparent's first cousin is a first cousin twice removed in relationship to the grandchildren. Some other relationships in families are second, third and fourth cousins. Second cousins have great-grandparents in common, while third cousins share great-great grandparents.
To figure out relationships in family trees, it is useful to have a chart or diagram. These charts are made up of rows and columns that represent the different relationships for at least two people in a family.