Question about a distribution list

I would like to know how to have a distribution list for each category of people I want to send a email to many people at one time. I want to put a list together of commercial brokers and commercial buyers. In groups, it is an open forum for each one to talk to and don't want that. How do I make lists and then when I start a new email, I would just put in lets say "commercial brokers" at the To line in the email and they would all get an email.