There are three ways to license SQL Server.  The top-line explanation
from the Microsoft web site:

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Processor license. Optimized for customers building
business-to-business (B2B), business-to-consumer (B2C), and
business-to-employee Web-based applications. A single license is
required for each CPU in the operating system instance running SQL
Server. This license does not require any device or user client access
licenses (CALs).

Server plus device CALs. Optimized for non-Web-based scenarios in
which there are multiple users per device (for example, kiosks). A
license is required for the computer running SQL Server 2005, as well
as a CAL for each client device.

Server plus user CALs. Optimized for non-Web-based scenarios in which
there are multiple devices per user (for example, a laptop, PDA, and
home PC). A license is required for the computer running SQL Server
2005, as well as a CAL for each user.
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Roy Harvey
Beacon Falls, CT