Many user accounts are used to control specific tasks. These accounts are special purpose. They have a user ID and belong to a group. They also have a password. These accounts are not meant for human beings though.
These accounts let a remote machine connect and establish a network connection using the PPP protocol. The remote system must identify itself using a standard chat sequence, sending its user name followed by a password.
These accounts are meant for authentication using the PAP protocol. The user name and password are exchanged using a special protocol defined in the PPP standard.
Same as for the PPP accounts above, except that a different, more secure, protocol is used to exchange authentication.
SLIP is another way of establishing a network connection between two computers over a serial line (modem).
UUCP is the Unix to Unix Communication Protocol. This is a batch oriented data transfer. It allows unattended exchange of files (and email) between two computers. It provides a very inexpensive but highly reliable way to automate data exchange. It is probably the best way (by far) to receive and send email on the internet unless you have a dedicated connection with your service provider.
See the networking section of Linuxconf to configure UUCP.
POP acccounts are restricted accounts. Users can't get a shell. They are generally used to retrieve email using the POP (Post Office Protocol). They can also be used to connect to other services such as Samba (file and print service for PCs machines).
These accounts are even more restricted. They allow retrieval of email using the POP protocol, from virtual email domains. See the menu "Networking/mail delivery system (sendmail)" to configure virtual email domains.