Many user accounts are used to control specific tasks. These accounts have special purposes. They have a user ID and belong to a group. They also have a password. These accounts are not meant for human beings, however; they are for system use.
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 username followed by a password.
These accounts are meant for authentication using the PAP protocol. The username and password are exchanged using a special protocol defined in the PPP standard.
Same as 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 (usually a modem).
UUCP is the Unix to Unix Communication Protocol. UUCP is used for batch-oriented data transfer. It allows unattended exchange of files (and e-mail) 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 e-mail on the Internet unless you have a dedicated connection to your service provider.
See the networking section of Linuxconf to configure UUCP.
POP accounts are restricted accounts. POP users can't get a shell. They are generally used to retrieve e-mail using the POP (Post Office Protocol). They can also be used to connect to other services such as Samba (file and print service for DOS/Windows machines).
These accounts are even more restricted. They allow retrieval of e-mail using the POP protocol, from virtual e-mail domains. See the menu "Networking/mail delivery system (sendmail)" to configure virtual e-mail domains.