Terminal Services CAL
The Terminal Services functionality is included with the acquisition of a Windows Server 2008 Server License. However, when a user or device accesses a terminal server, that user or device is required to have a TS CAL (in addition to a Windows CAL) in order to be compliant with the licensing terms of the product. A separate TS CAL is required for each user or device that accesses or uses the server software on any server.

User CAL
A Windows Server User CAL permits one user (using any device) to access or use the server software. User CALs are likely to be more cost effective than Device CALs if there are multiple devices per user (for example, a user has a laptop and handheld computer accessing Terminal Services)