What is the difference between a Windows Server CAL and a Remote Desktop Services (RDS) CAL?

A Windows Server CAL is required when a user or device is directly or indirectly accessing a Windows server. Additionally, if the user or device is accessing or using the terminal services functionality of Windows Server, a RDS CAL is also required. As an exception to these rules, up to two users or devices may access the server software only for server administration purposes, without requiring either a RDS CAL or Windows Server CAL.