Roles
Roles control permissions to take actions on the platform. Every member in your community will have a role so Heartbeat knows what they are allowed to manage in the community. For example, the most common 3 roles used in communities are Admin, Moderator, and User. Your members will all most likely be users. You can check out all the permissions you can control for each of these roles in Admin Settings > Roles.
Groups
Groups control access to content in your community (courses, docs, discussion channels, matchups, events, etc.). You'll create groups for your 3 tiers of customers since their membership to a tier will determine what they see and consume in the community.