Club members are those people who have joined your club either for free or for a monthly fee that you set. Club members have access to the content you post. Free members can only view free content. Paid members can view all the content you post.
Followers are those who chose to just follow you but do not join your club. Followers cannot see any of your content, but your club (profile) shows up in their Memberships section. They can choose to join your club at any point.
Followers, free members, and paid members also appear as groups that you can target with your mass messages.