A virtual package is a generic name that applies to any one of a group of packages, all of which provide similar basic functionality. For example, both the tin and trn programs are news readers, and should therefore satisfy any dependency of a program that required a news reader on a system, in order to work or to be useful. They are therefore both said to provide the “virtual package” called news-reader.
Similarly, smail and sendmail both provide the functionality of a mail transport agent. They are therefore said to provide the virtual package, “mail transport agent”. If either one is installed, then any program depending on the installation of a mail-transport-agent will be satisfied by the existence of this virtual package.