cardinality

Relationships between data tables define cardinality when explaining how each table links to another.
In the relational model, tables can be related as any of: many-to-many, many-to-one (rev. one-to-many), or one-to-one. This is said to be the cardinality of a given table in relation to another.

List all groups and users in Linux

cat /etc/groups  -- Lists all the users in the current system
cat /etc/passwd -- Lists all users