A content contributor is the typical user account for Savvy Content Manager. They are a user who has the ability to update information within the site in some way. They have user options that range from editing, publishing, creating pages, etc. Before version 4.5 all user accounts were content contributors.
They differ from members in that a member can't do anything but log in and see content that they have permission to view.
Only Content Contributor users count against your licensed number of users allowed.