Excerpt from Microsoft.com:
… By having two types of Windows CALs, you are able to use the model that makes sense for your organization. For example, purchasing a Windows User CAL might make more sense if your company has a need for employees to have roaming access using multiple devices. Windows Device CALs may make more sense if your company has multiple-shift workers who share devices. Similarly, Terminal Server (TS) will offer both device-based and user-based CALs: TS Device CAL and TS User CAL.
In summary, User CAL (Unlimited Device under One User), Device CAL (Unlimited Users under One Device).
As of this posting date, the per CAL MSRP for both types of CAL’s are the same.