I'm asking because I often see so many books told me to use mail() feely, but I don't really understand.
Can we really use this without even have a mail server or anything related to that?

Or even if we need something before using this function, can anyone tell me waht those are?

Right now my website had not have this function yet, but I'm trying to, because it's necessary sometimes when I got another project.

the MAIL function does need an SMTP server to be active as thats how it works. T17 has this server so you can use MAIL without any setup needed on T17.

this page described the function http://uk2.php.net/manual/en/function.mail.php

The advantage to using MAIL comes from being able to automatically send emails such as n user registration or newsletters. I think there was a discussions about how to send mail without a mail server but youd have to search for that.

It is a very useful function so use it wisely!