Mass mailing is the practice of sending large amounts of non-solicited email. A tool used to reach a large amount of users in one hit.
Mass mailing is when you send any kind of group email. Any email that is sent out and generates a complaint is consider Spam.
There is no full proof way to avoid spamming when mass mailing, however you should always ensure you are using email addresses that you have collected yourself or use a thrid party service.