Unlike other operating systems, Linux manages security problems, and especially user groups.
For Linux, there are three types of users: yourself, the group you belong to, and others. Each user of the machine owns its own personal directory, which is a subdirectory of the /home directory. For example, if your login name is ``fred'', your personal directory (``home directory'') will be /home/fred (you will often see ``~'' which symbolises your personal directory under a Unix shell). The only exception is the ``root'' account the home directory of which is /root. This concept of personal directory enables you to protect your sensitive data from any other user (only the super-user (root) has access to all the users directories).
Nevertheless, you may certainly want to share some given documents with other users. This is why Linux implements the ``group'' notion. A group is a set of users. Any user belongs to a default group, but it is possible to belong to multiple groups. The system administrator must organize the groups in a logical way. Frequently, a group is created for each function: one group for the development team, one other group for the commercial staff etc.