What is the mail message exactly means ?

What is the mail message exactly means ?

Is it just limited to sending messages to only site members

OR

is there a way to send email to actual email ids like gmail, yahoo etc.

And if it is just limited to sending messages between site members I would like to rename that function from mail messages to just messages.

Can we send mails to actual mails, like yahoo, gmail, hotmail, etc........  ???