Workbooks enables you to identify People and/or Leads on your database and group them together in a Mailing List so they can all be sent the same email en masse.

Mailing Lists can be created manually but more commonly they're created from a Report that targets a particular group of People/Leads with a shared attribute (or attributes) or are created from the membership of a Marketing Campaign.

Clicking on Start > Marketing > Mailing Lists displays the Mailing Lists Landing Page where you can view existing Mailing Lists.

To open a Mailing List hover over it until it's underlined and click on it.