The /etc/passwd is a colon separated list of attributes of a user account. A very similar file is /etc/shadow. The /etc/passwd file has the following fields. 

• Field 1 = username
• Field 2 = password
• Field 3 = numeric user ID (UID)
• Field 4 = numeric default group ID (GID)
• Field 5 = Comment (empty by default)
• Field 6 = Users home directory (e.g. /home/john.doe)
• Field 7 = Users default shell (e.g. /bin/bash)

For example, let's say one of the lines in /etc/passwd contains the following.

• Field 1 = username = john.doe (The usermod command can be used to change the username)
• Field 2 = password = x to indicate the user has a password (the password is not "x") - passwords are stored in /etc/shadow. The passwd command can be used to create a new encrypted password. The usermod -p command can be used to create a new cleartext password.
• Field 3 = numeric user ID = 1002 (The usermod -u command can be used to change the UID)
• Field 4 = numeric default group ID = 2002 (The usermod -g command can be used to change the GID)
• Field 5 = Comment = empty / no comment (The usermod -c command can be used to add or change the comment)
• Field 6 = Users home directory = /home/john.doe (The usermod -d command can be used to change the home directory)
• Field 7 = Users default shell = /bin/bash (The usermod -s command can be used to change the default shell)

john.doe:x:1002:2002::/home/john.doe:/bin/bash

There are a few commands that can be used to determine if a user account exists.

• The cat command to read the /etc/passwd file (this article)
• The id command
• The getent passwd command