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Microsoft offers two ways of accessing SQL Server. You can go with the CALs model or you can go with the per processor (also known as CPU) model.
You will find these models in SQL Server 2000 Standard, SQL Server 2000 Enterprise, SQL Server 2005 Standard, SQL Server 2005 Enterprise and SQL Server 2005 Workgroup.
 Processor license Requires a single license for each processor (also known as CPU) that the underlying Windows operating system is accessing. The Processor license model eliminates the need for CALs (Client Access Licenses).
To find out more about the relationship between the number of processors and the number of licenses, click on the link How many Processor Licenses? in the Product Info box on the right.
Client Access Licenses With this model, you need a license for running SQL Server, and additional CALs (Client Access Licenses) for each device or user that will at some time be accessing SQL Server.
Microsoft don't offer concurrent licenses. This means that if you have, say, 50 users who will be accessing SQL Server, but only ten users will be accessing SQL Server at any given time, you have to buy 50 CALs. Buyng only ten CALs won't be sufficient.
Choosing between CALs and Processor licenses
From a financial view, as a rule of thumb, if you are looking to buy 25 or more CALs, you should seriously consider the Processor license. Less than that and it is the CAL model that makes financial sense. This applies to the Standard Edition. For the Enterprise Edition, generally the figure is 80 or more CALs.
Of course, this assumes that the Windows operating system that SQL Server is sitting on is only accessing one processor. If it is accessing two processors, you will need two Processor licenses. At which point, the rule of thumb changes to having to buy 50 or more CALs for Standard and 160 or more CALs for Enterprise before the per processor model becomes financially preferable.
For web applications, where you don't know how many users will be accessing the SQL Server, the only option is the Processor model.
An advantage to the Processor licensing model is that it eliminates the need to count devices or users, which many administrators find cumbersome and difficult to manage on an ongoing basis.
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