I am going to disagree with DarkSheep, As much as it would be nice to have a good unix-wide distinction between users and accounts (bob is a user, apache is an account) this is not a distinction that really exists outside a small number of administrative tools. The confusion between users (Bob, a man who has less hair than he wishes), user accounts (bob, the login for the man who wants more hair), pseudo-users (the thing the computer thinks is out there, maybe), and pseudo-user accounts (the user account set aside for the web server) is not well defined on a truly global scale (although some distributions are trying to draw a line in the sand)
If we were to adopt site specific definitions (which I think would be a good thing as long as we recognise that these definitions are not fully accepted) I would propose the following:
System User Account: The association of data identifying a user returned by
pwgent including UID, GID, username, and related information, but specifically not including password related information.
Application User Account: any user account used by an application that is not returned by pwgent.
Pseudo-user Account: Any special case of System User Account that does not have a corresponding User.
Regular User Account: Any System User Account associated either with an individual or organization.
Don't even get me started on groups Which are an even bigger mess with:
Feel free to improve this.