A group is a set of users.group information is stored in the /etc/group directory.it store information about a group in one line with different field separated by comma,these fields are group name,group id,password,users,these user created that time it takes default group,the method of assigning a default group is called user private group scheme.
You can manage group using either shell commands or GUI utilities.group are an effective way to manage access and permissions,letting you control several users with just thir group name.
The system file that holds group entries is called /etc/group.the file consists of group records,with one record per line and its fields separated by colons.a group record has four fields: a group name, a password,its ID,and the users who belongs to this group.the password field can be left blank.
Group Name: The name of the group,which must be unique.
Password: With security implemented,this field is an x,with the password indicated in the /etc/gshadow file.
Group ID:The number assigned by the system to identify this group.
Users:The list of users that belong to the group,separated by commas.
Here ia an example of an entry in an /etc/group file.the group is called 'root',the password is managed by shadow security,the group ID is 0 ,and the users who are part of this group are group1,group2,group3 and group4.
A User Private Group is created whenever a new user is added to the system.a user private group has the same name as the user for which it was created and that user is the only member of the UPG.user private group make it safe to set default permission for a newly created file or directory which allow both user and that users group to make modifications to the file or directory.(Traditionally,user were all assigned to one group named users that subjected all users has its own group,with its own group permission.).