Lists are a collection of Contacts grouped together and given a name. They allow you to group similar Contacts to make it easier to focus your marketing efforts on common themes, preferences and needs.
If you have a Marketing Integration (like Campaign Monitor) linked to GlassHive, these lists will also appear in Campaign Monitor. This makes it extremely easy to create campaigns that target specific Lists created in GlassHive.
Lists also determine which Contacts are considered Leads. If a Contact is part of a List that has been marked as a Leads List, it means those Contacts can potentially become Leads.