How to add "system" local user like mysql or tomcat?

My system has many users, I've never created, like mysql or tomcat. These users have no home directories inside /home Obviously, a daemon program runs under these users. What is the term for such users? How to create such a user of my own? For example, I wish to create a user for deluged, but I don't want to create and remember a password for it and also don't want to allow somebody to login with this user from console. How to accomplish this?