Here's a great tutorial on how it works:
http://support.canadawebhosting.com/KB/kbSearch.asp?KBID=1446

To access the control panel you just saw on the tutorial go to http://yourdomain.com/admin

Your login is the same as your website.

When you want to send an email to the subscribers, go to:
http://yourdomain.com/squirrelmail (This is your web based mail server)
Log in:
youraccountname@yourdomain.com
Password: same as for your website

Open compose email: 
To: mynews@mydomain.com - where "mynews" is the mailing list you created in your control panel (see tutorial above.)

- Then write whatever you want in the body and send it.

That's all there is to it. In this manner, you have a powerful communications tool that allows you to inform your subscribers of any updates on the website or your discussion group.