User accounts are the primary means of accessing your server. For security and tracking purposes, you should give each person with access to your server a unique user account. User accounts insulate users from one another, so that one user cannot affect another user or perform an action that may damage the system. User accounts also allow you to find out who is doing what, to prevent one user from reading mail meant for another, and so forth.
Your server provides three different types of user accounts. When you add a user to your system, you can give that user access to email only, to email and FTP, or to email, FTP and a shell.