Ubuntu Server: How to list all the groups and users

Linux is a multi-user operating system.  This means that the administrator will have to be careful in how users are managed. List of users is maintained in /etc/passwd along with more information like the group the belong to etc. To view its contents, run: cat /etc/passwd
The each line of output is like:
www-data:x:33:33:www-data:/var/www:/bin/sh
Here there are 7 items mentioned below in order from left to right:

1. Username: It is used when user logs in. It should be between 1 and 32 characters in length.
2. Password: An x character indicates that encrypted password is stored in /etc/shadow file.
3. User ID (UID): Each user must be assigned a user ID (UID). UID 0 (zero) is reserved for root and UIDs 1-99 are reserved for other predefined accounts. Further UID 100-999 are reserved by system for administrative and system accounts/groups.
4. Group ID (GID): The primary group ID (stored in /etc/group file)
5. User ID Info: The comment field. It allow you to add extra information about the users such as user's full name, phone number etc. This field use by finger command.
6. Home directory: The absolute path to the directory the user will be in when they log in. If this directory does not exists then users directory becomes /
7. Command/shell: The absolute path of a command or shell (/bin/bash). Typically, this is a shell. Please note that it does not have to be a shell.

 In short: username:password:UID:GID:info:home directory:shell


Further, If you want to check the groups to which a given user belongs, run: groups
Or to check members of a group, run member
(members command is not installed by default. Run sudo apt-get install members to install)

Source:

• Ubuntu 10.04: Managing Users 
• /etc/passwd Format
• how-to-list-users-in-a-group-on-ubuntu-server

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