In Unix, what is a group?

In Unix, a group is a logical collection of users on a system, the primary use of which is to assign "group ownership" of files and directories. As a result, certain groups of users on a system can all have the same access rights to the designated files and directories. Each group is independent of other groups and there is no specific relationship between groups. For a list of the groups to which you belong, at the Unix prompt, enter:

groups
To change the group ownership of a file, use the chgrp command. Typically, you can only change the file's group to a group to which you belong. For more information, read the chgrp man page by entering:

man chgrp