Using an alias as a mail address allows you to send a message to several people using only a single address.
To creat an alias, define it in the .mailrc file in your home directory. For example:
alias unixpeeps keith@castle rob@castle\ paul@castle tony@castle
This creates a single alias for the addresses of four users. Using the alias unixpeeps as the mail address will send the message to each one of them.
You can also use an alias within an alias, and so on!