/etc/passwd

This file stores information about users.

Each line in /etc/passwd is in this format:

username:has_pw:userid:group:comment:homedir:shell

The fields are separated by colons and include:

username
The user's login name. This can contain any characters except a colon (:), but you should probably avoid any of the shell's special characters. For more information, see Quoting special characters in Using the Command Line.
has_pw
This field must be empty or x. If empty, the user has no password; if x, the user's hashed password is in /etc/shadow.
userid
The numeric user ID.
group
The numeric group ID.
comment
A free-form comment field that usually contains at least the user's real name; this field must not contain a colon.
homedir
The user's home directory.
shell
The initial command to start after login. The default is /bin/sh.
Note: You can't specify any arguments to the login program.

Here's a sample entry from /etc/passwd:

fred:x:290:120:Fred L. Jones:/home/fred:/bin/sh