Site Database Server : This is a server with Microsoft SQL Server installed, hosting the ConfigMgr site database.
Site Server : A site server is a computer system where Configuration Manager 2012 or 2007 is installed. This main role contains components and services required to run a central administration, primary, or secondary site.
Site System : This role supports both required and optional site system roles. Any server (or share) with an assigned role automatically receives this role. For Example Similar to a Windows Server role. SUP, DP, MP, Asset Intelligence sync point, etc.
